Saccade Reorienting Is Facilitated by Pausing the - Am MIT spezialisierte KI-Forschung

Saccade Reorienting Is Facilitated by Pausing the
Oculomotor Program

Antimo Buonocore1, Simran Purokayastha2, and Robert D. McIntosh2

Abstrakt

■ As we look around the world, selecting our targets, compet-
ing events may occur at other locations. Depending on current
Ziele, the viewer must decide whether to look at new events or
to ignore them. Two experimental paradigms formalize these
response options: double-step saccades and saccadic inhibition.
In the first, the viewer must reorient to a newly appearing tar-
get; in the second, they must ignore it. Until now, the relation-
ship between reorienting and inhibition has been unexplored.

In three experiments, we found saccadic inhibition ∼100 msec
after a new target onset, regardless of the task instruction.
Darüber hinaus, if this automatic inhibition is boosted by an irrele-
vant flash, reorienting is facilitated, suggesting that saccadic in-
hibition plays a crucial role in visual behavior, as a bottom–up
brake that buys the time needed for decisional processes to act.
Saccadic inhibition may be a ubiquitous pause signal that pro-
vides the flexibility for voluntary behavior to emerge. ■

EINFÜHRUNG

As we look around the world, selecting the targets of our
eye movements, the scene may change. Our next planned
target may move or an event of greater urgency may occur
at another location. Depending on current goals, Die
viewer must decide whether to look at new events or to
ignore them. These response options have been formal-
ized within two powerful experimental paradigms: double-
step saccades and saccadic inhibition. In the first, Die
viewer must reorient to a target appearing shortly before
a planned saccade; in the second, they must ignore it.
Each paradigm has been studied extensively, but the rela-
tionship between reorienting and inhibition is hitherto
unexplored. This study seeks to bridge that gap, testing
the presence and possible functional role of oculomotor
distraction in reorienting.

Irrelevant distractors robustly delay saccade execution
(z.B., Walker, Kentridge, & Findlay, 1995; Weber & Fischer,
1994). Analysis of latency distributions reveals that distrac-
tors cause a specific dip in saccadic activity ∼60–120 msec
after distractor onset, with the suppressed saccades launch-
ing later (Bompas & Sumner, 2011; Edelman & Xu, 2009;
Buonocore & McIntosh, 2008). This pause in visual be-
havior, known as “saccadic inhibition,” is a fast, reflexive
response of the oculomotor system to any salient visual
ändern, and it occurs regardless of the task instruction
(Reingold & Stampe, 1999, 2002, 2003, 2004). We should
thus expect that saccadic inhibition will accompany the
change in target position in double-step tasks.

1Tübingen University, 2University of Edinburgh

The literature on double-step saccadic performance
contains hints that this may be the case. Allgemein, sac-
cades launching immediately after a target step do not
have time to reflect the step and are directed toward
the initial target, whereas saccades launching at longer
delays are progressively more likely to go toward the sec-
ond target (Becker & Jürgens, 1979; Lisberger, Fuchs,
King, & Evinger, 1975). At least for large target steps, Das
“transition function” is discontinuous, with saccades
launched less than ∼60 msec after the target step going to
the initial target, saccades launched more than ∼120 msec
after the step going to the new target, and few if any
saccades in the intervening time window (Aslin & Shea,
1987; Findlay & Harris, 1984). There is thus a pause in
behavior in double-step tasks that corresponds roughly
with the time course of saccadic inhibition. Speculation
that the target step in double-step tasks has a distract-
ing effect can be found quite far back in the literature
(Sheliga, Braun, & Miles, 2002; Findlay & Walker, 1999),
yet this important possibility has never been formally
examined.

The first aim of this study is to assess whether the sig-
nature of saccadic inhibition is present in a double-step
Kontext, comparing between two tasks, matched for stim-
ulus events, in which a new visual event is to be responded
to as a target or ignored as irrelevant. We demonstrate
that saccadic inhibition does indeed occur in a double-
step context, just as in a distractor context. Experiment 3
is then designed to study the consequences of this reflex-
ive inhibition for reorienting behavior. This is done by
boosting the inhibitory signal during double-step reorient-
ing, with a salient generalized flash presented at the
same time as the target jump. One might expect that this

© 2017 Massachusetts Institute of Technology. Published under a
Creative Commons Atrribution 3.0 Unportiert (CC BY 3.0) Lizenz.

Zeitschrift für kognitive Neurowissenschaften 29:12, S. 2068–2080
doi:10.1162/jocn_a_01179

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irrelevant event would interfere with the ability to respond
to the new target; but a more interesting, counterintuitive
possibility is that the flash will interfere mainly with the
planned response to the initial target, creating a pause
in behavior that would increase the subsequent likelihood
of successful reorienting. Wenn ja, we would predict that
reorienting would be enhanced under conditions of
boosted inhibition, indexed by an increased proportion
of saccades being successfully redirected. This would pro-
vide strong support for a long-standing but little-discussed
idea that saccadic inhibition is a feature, not a bug of the
oculomotor system, which buys time for the system to
evaluate any salient change and to alter the next saccade
if necessary (Reingold & Stampe, 2002).

METHODEN

Teilnehmer

Eighteen volunteers, aged between 18 Und 30 Jahre, Par-
ticipated in the three experiments (six in each). Das
study is concerned with basic design features of the
oculomotor system, so we are targeting only very consis-
tent effects that should be near-universal. The number of
participants per experiment is therefore small (n = 6),
with effort directed to maximizing the number of obser-
vations per participant to ensure robust individual param-
eter estimates. Our focus is on a high consistency of
behavior across participants, and individual parameters
are reported for each participant in each experiment.
All were free from neurological and visual impairments.
This experiment was conducted in accordance with the
1964 Declaration of Helsinki, with the approval of the
University of Edinburgh Psychology research ethics
committee.

Apparatus, Stimuli, and Procedure

Stimuli were black on a mid-gray background, vorgeführt
on a 19-in. CRT monitor (1024 × 768 pixels) bei 100 Hz.
All the experiments were implemented in Experiment
Builder (SR Research, Ottawa, ON, Kanada). Teilnehmer
were seated with their head on a chinrest, their eyes
aligned with the center of the screen at a distance of
80 cm. Eye movements were recorded with the EyeLink
1000 System (SR Research; detection algorithm: pupil
and corneal reflex; 1,000 Hz sampling; saccade detection
based on 30°/s velocity and 8,000°/s2 acceleration thresh-
olds). Each trial began with drift correction and a tone
followed by a 0.5° central fixation cross. In Experiments
1 Und 2 (but not Experiment 3), 1° black outline circles
were also presented at 6° to left and right of fixation as
placeholders for potential targets. In all experiments, A
5-point calibration on the horizontal and vertical axes
was run at the beginning of each session and after three
consecutive blocks. Additional calibrations were run if
the participant’s head moved from the chinrest.

Experiment 1 (Figure 1A)

Each trial began with fixation followed by the onset of
Target 1, created by filling in one of the two lateralized
outline circles. In half of the trials, Target 1 remained
filled until the screen went gray 700 msec later (target-
only trials). In the other half of the trials, Target 1 unfilled
and the outline on the opposite side filled simultaneously
(Target 2), after a delay determined individually for each
participant (see Preliminary Block section). In the “dis-
tractor” condition, participants were required to ignore
Target 2 and to move their eyes to the Target 1 location.
In the “step” condition, participants were required to
move their eyes to the second target if it appeared.

Each participant completed two sessions for the dis-
tractor and step conditions,1 following an ABBA se-
quence, counterbalanced across participants. Within
each session, there were 10 blocks of 24 Versuche; Target 1
appeared equally often on each side and, for each side,
was followed by Target 2 in half of the trials. Teilnehmer
thus completed 240 target-only and 240 distractor trials
in the distractor condition and 240 target-only and 240 step
trials in the step condition.

Experiment 2 (Figure 1B)

This was identical to Experiment 1, except that Target 1
did not unfill when Target 2 was presented. Stattdessen, für
the trials in which Target 2 appeared, both targets re-
mained filled until the end of the trial. Daher, wohingegen
Experiment 1 used stimulus conditions typical of double-
step tasks (Target 1 replaced by Target 2), Experiment 2
used conditions more typical of distractor tasks (Target 1
joined by Target 2).

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Figur 1. Schematic diagram of trial events. See Methods for details.

Buonocore, Purokayastha, and McIntosh

2069

Experiment 3 (Abbildung 1C)

Saccadic Inhibition Analysis

This was similar to Experiments 1 Und 2, with some im-
portant changes. After the initial fixation period, a 1°
black dot appeared on the left or the right (Target 1).
After the delay determined individually for each par-
ticipant (see Preliminary Block section), one or two
changes could occur: In flash trials (25% of trials), Dort
was a distracting flash of maximum contrast, dauerhaft
30 msec and covering the top and bottom thirds of
the screen; in step trials (25% of trials), Target 1 dis-
appeared and was replaced by an identical target on
the opposite side (Target 2); in step-flash trials (25% von
Versuche), both changes happened simultaneously; in target-
only trials (25% of trials), neither change happened.
In all trials, the participant was required to move their
eyes as rapidly as possible to the target, thus to change
plan if the target stepped. Note that the flash condition
of Experiment 3 is similar to the distractor conditions of
Experimente 1 Und 2, except that the distracting event is
a large generalized flash.

Each participant completed two sessions of 10 blocks
jede. Within each block, there were 48 Versuche; Target 1
appeared equally often on the left and the right, Und
for each target side, there were six trials in each of the
stimulus conditions (target-only, flash, step, step-flash).
Participants thus completed 240 trials per condition.

Preliminary Block

Before each experiment, participants performed a block
von 70 trials of the target-only condition for that experi-
ment. The median saccadic RT of the last 50 trials was
berechnet, Und 100 msec was subtracted from this value
to set the timing of transient events for that participant in
the subsequent experiment. These participant-specific
timings ensure that saccadic inhibition, which is maximal
around 90–100 msec after a transient event, will impact
upon a dense portion of the expected saccadic distribution.
The individual transient onset times relative to initial target
onset are listed per participant: Experiment 1: 150, 150,
120, 120, 90, 170; Experiment 2: 120, 90, 140, 130, 150,
140; Experiment 3: 20, 120, 120, 170, 130, 80.

Data Processing

All the data processing and analysis were performed in
MATLAB (The MathWorks, Inc., Natick, MA). Only the
first eye movement following target onset was analyzed.
A total of 9% (Experiment 1), 2.7% (Experiment 2), Und
4.3% (Experiment 3) of trials were excluded from the
analysis due to blinks, latencies shorter than 70 ms
or longer than 500 ms, and saccades smaller than
2° amplitude or with a duration exceeding 100 ms. Für
the target-only condition, we also excluded saccades made
in the wrong direction (Experiment 1: 0.67%, Experiment 2:
1.31%, Experiment 3: 0.21%).

To chart the time course of saccadic activity, we applied a
distribution analysis to the saccadic RTs (vgl. McIntosh &
Buonocore, 2014; Bompas & Sumner, 2011; Reingold &
Stampe, 2002). In each experiment, für jeden Teilnehmer
and condition, we computed the probability density esti-
mate of saccades at each millisecond, with time zero de-
fined by the transient change. The estimate was smoothed
using a kernel-smoothing window of 8 ms. For each par-
ticipant, for each experiment, the target-only condition was
taken as the baseline distribution of saccadic activity. Wir
berechnet, at each millisecond, the proportional change
relative to this baseline in each of the transient change
Bedingungen (distractor or step in Experiments 1 Und 2;
flash, step, or step-flash in Experiment 3) using Equation 1,
which expresses reduced saccadic activity as positive:

ptarget tð Þ − pchange tð Þ
ptarget tð Þ

(1)

where p(T) is the probability for the target-only condition
und p(C) is the probability for the transient change con-
dition. To keep the estimate of proportional change reli-
able, considering the low frequency of saccades in the
initial and final part of the distribution, we extracted
data only from 45 Zu 250 msec after the change. Aus
each of these profiles, we extracted inhibition magnitude
(maximum inhibition), inhibition latency (time to maxi-
mum inhibition), and inhibition start and end times,
defined operationally as the times at which inhibition
crossed 50% of its maximum level, before and after the
maximum, jeweils (Reingold & Stampe, 2002).

Double-step Analysis

To characterize reorienting behavior for each condition
that had a “step” (d.h., in which the participant was re-
quired to reorient to Target 2), we derived a direction
transition function. Erste, we classified each saccade as
directed at Target 1 or Target 2, according to the hemi-
field in which it landed (the distribution of landing posi-
tions was bimodal, as is clear in Figures 2, 4, Und 6). Wir
then computed the probability of responding to Target 1
as function of time since the step and fitted a logistic
function using Equation 2:

D
1 þ e

−a * t−b
D

(2)

where e is the natural logarithm base, a represents the
steepness of the curve, b is the inflection point, Und
the numerator represents the maximum probability.

Probability of Responses to Target 1

In each experiment, for each condition, we estimated
the proportion of responses to Target 1, with Target 1
responses coded as 1 and Target 2 responses coded as 0.

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Figur 2. Each panel shows the probability density estimate derived from the raw data of each participant (P1–P6) in the distractor (Rot) and step
(Blau) conditions of Experiment 1. The horizontal components of the saccade landing positions are overlaid (negative values indicate Target 1
location, and positive values indicate Target 2 location). Time on the x axis is coded relative to transient onset (d.h., positive numbers indicate
saccades launched after distractor onset), rather than initial target onset to reveal the time course of responses to the transient change.

In distractor and flash conditions, Teilnehmer waren
required to respond to Target 1, so the expected pro-
bability is very high. Umgekehrt, in step and step-flash
Bedingungen, participants had to reorient to Target 2, mak-
ing the expected probability to Target 1 lower, to the
extent that the participant was able to reorient. We were
most interested in these proportions for step and step-
flash conditions in Experiment 3, in which lower num-
bers indicate more reliable reorienting (d.h., lower error
rate).

Reorienting Latency

For conditions involving a step, we also estimated the
average latency of successful reorienting responses, als
the median saccadic RT for saccades to Target 2, coded
relative to the target step.

Statistical Analysis

Experimente 1 Und 2 were designed as exploratory stud-
ies to assess whether the signature of saccadic inhibition

was present in a double-step context. There were no
critical hypotheses for these two experiments, so formal
statistical tests were not used. Experiment 3 was then
designed to test whether saccadic inhibition facilitated
reorienting. For this experiment, we ran a series of pair-
wise t-test comparisons to test differences among param-
eters of interest. Given the low number of participants
(N = 6), which makes parametric assumptions hard to
verify, we also ran nonparametric statistics. We report
the parametric analysis in the Results section, but all non-
parametric comparisons had similar outcomes and do
not modify our findings.

ERGEBNISSE

Experimente 1 Und 2 were designed to provide a descrip-
tive comparison of saccadic activity in distraction and
double-step tasks. We expected to observe the classic re-
duction in saccade frequency around 100 msec after the
transient event. Experiment 3 was designed to test the
influence of saccadic inhibition upon measures of re-
orienting and the influence of the reorienting instruction

Buonocore, Purokayastha, and McIntosh

2071

upon measures of inhibition. Formal statistical tests are
reported for Experiment 3 nur.

probability of Target 1 = 0.26, SD = 0.1), selbst nach
the maximum of inhibition.

Experiment 1

Figur 2 shows the saccadic RT profiles for each partici-
keuchen, showing the probability of saccade launching by
time after transient onset. Saccadic inhibition is indicated
by the bimodality of these distributions; its consistency
across participants is striking, with a depression of sac-
cadic activity centered around 100 msec after the tran-
sient change (Buonocore, McIntosh, & Melcher, 2016;
Buonocore & McIntosh 2008, 2012, 2013; Bompas &
Sumner, 2011; Edelman & Xu 2009; Reingold & Stampe,
1999, 2000, 2002, 2004). Even more striking is that this
bimodal pattern emerged regardless of whether the par-
ticipant was required to ignore or to reorient to a target
step. The data show, for the first time, that the disconti-
nuity of saccadic behavior in double-step reorienting
(Ludwig, Mildinhall, & Gilchrist, 2007; Findlay & Harris,
1984; Becker & Jürgens, 1979) closely matches the char-
acteristic time course of saccadic inhibition.

The horizontal components of the saccade landing
positions are overlaid in each panel of Figure 2. For the
step condition, these clouds of points cluster to Target 1
or Target 2 along the time dimension, reflecting the
double-step transition. Virtually no eye movements to-
ward Target 2 were launched before the start of the inhi-
bition. In all participants bar one (P02), reorienting was
permeated by erroneous saccades to Target 1 (mean

The group average time course of inhibition derived
from the saccadic inhibition analysis for each condition
is shown in Figure 3, and the individual parameters are
shown in Table 1. Inhibition starts around 81 ms, mit
saccadic activity driven almost to zero by 119 ms, Re-
turning to normal levels by 165 ms. The overlaid group
average direction transition function for the step condi-
tion implies that the transition is parallel to the time
course of saccadic inhibition, with its inflection point
on average ∼16 msec after the peak of inhibition (sehen
Tisch 2 for individual parameters).

Experiment 1 thus demonstrates saccadic inhibition in
a double-step context and further shows a temporal over-
lap of saccadic inhibition and reorienting. Experiment 2
asks whether the pattern holds under stimulus condi-
tions more similar to a classic distractor paradigm, In
which Target 1 does not disappear with the appearance
of Target 2 but persists (Buonocore & McIntosh, 2008;
Walker et al., 1995).

Experiment 2

Figur 4 shows the distributions for each participant,
with horizontal landing positions overlaid. As in Experi-
ment 1, there is clear impression of bimodality consistent
with saccadic inhibition in distractor and step conditions;
this is, if anything, more pronounced in the step condi-
tion. The landing positions show that erroneous saccades

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Figur 3. Group average time course of proportional inhibition derived from the saccadic inhibition analysis for distractor (Rot) and step (Blau)
conditions of Experiment 1. For the step condition, the black line represents the group average direction transition function coded as the proportion
of saccades directed to Target 1 (right y axis). Time on the x axis is coded relative to transient onset (d.h., positive numbers indicate saccades
launched after distractor onset), rather than initial target onset to reveal the time course of responses to the transient change.

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Buonocore, Purokayastha, and McIntosh

2073

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Tisch 2. Individual Parameters, Means, and Standard Deviations for Direction Transition Function (DTF) and Other Measures of
Reorienting Behavior for Each Experiment and Condition

Experiment

Participant

Schritt

Step-flash

Schritt

Step-flash Distractor

Schritt

Step-flash

Schritt

Step-flash

DTF Slope

DTF Inflection

Probability T1

Reorienting Latency

Mean

−0.90

−0.09

−0.02

−0.06

−0.06
−0.20

0.35
−0.05

−0.05

−3.30

−0.07

−0.05

−0.06
−0.60

1.32
−0.11

−0.86

−0.26

−1.74

−1.51

−1.49
−1.00

–

−1.34

−3.10

−0.14

−0.34

−1.01

−0.29
−1.04

108

152

149

152

162

135

29.69

165

152

109

145

138

185

149

25.71

116

135

126

136

121

127

–

108

154

124

115

152

146

133

0.69

1.11

7.80

19.91

0.89

0.91

0.80

0.94

0.69

0.90

0.86

0.10

0.94

0.92

0.99

0.98

0.99

0.96

0.03

1.00

0.99

1.00

0.01

0.26

0.36

0.37

0.12

0.18

0.28

0.26

0.10

0.38

0.33

0.23

0.37

0.29

0.33

0.06

0.10

0.44

0.65

0.51

0.61

0.14

0.41

0.24

–

0.01

0.34

0.51

0.44

0.47

0.08

0.31

0.21

208

217

222

220

223

251

223

14.34

205

199

162

188

199

243

199

26.30

220

256

188

239

203

202

218

–

219

278

211

250

242

198

233

25.62

29.31

to Target 1 (failures of reorienting) were relatively common
in the step condition (mean probability of Target 1 = 0.33,
SD = 0.06). Noch einmal, virtually no reorienting saccades
to Target 2 were launched before the start of inhibition.

In Abbildung 5, the group average transition function for
the step condition is overlaid on the average profile from
the saccadic inhibition analysis and alongside the inhi-
bition profile for the distractor condition. Da ist ein
broad superimposition of the temporal profiles of inhi-
bition and reorienting, and the point of inflection in
the transition function lags the peak of inhibition (von
∼34 msec). Jedoch, visual inspection suggests that
the transition function now extends further beyond the
end of the inhibition profile than in Experiment 1. Das
is not due to a change in the direction transition function,
which is closely similar between Experiments 1 Und 2, Aber

instead to alterations in the inhibition profile. Descriptively,
the magnitude of the inhibition is reduced in Experiment 2
(due to the persistence of Target 1), and this is associated
with a weaker and contracted inhibitory profile, a lower
maximum inhibition being reached relatively sooner, Und
an inhibition ending earlier (the numerical differences
between these parameters can be appreciated in Table 1).
Comparing inhibition profiles between the step and
distractor condition of Experiment 2 (Figur 5 Und
Tisch 1), the onset times are similar, but the maximum
inhibition is stronger (in every participant) and often
ends later (in five of six participants) in the step than in
the distractor condition of Experiment 2. This may sug-
gest that the bottom–up inhibition associated with the
target step accounts for only a proportion of its inhibitory
effect in the step condition, having its main influence in

2074

Zeitschrift für kognitive Neurowissenschaften

Volumen 29, Nummer 12

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Figur 4. Each panel shows the probability density estimate derived from the raw data of each participant (P1–P6) in the distractor (Rot) Und
step (Blau) conditions of Experiment 2. The horizontal components of the saccade landing positions are overlaid. Time on the x axis is coded relative
to transient onset, rather than initial target onset, to reveal the time course of responses to the transient change.

the early part of the profile, and that the latter part of
the profile is additionally shaped by reorienting behavior.
Experiment 2 thus replicates the finding of saccadic in-
hibition in a double-step context, but additionally sug-
gests some possible dissociability from the time course
of reorienting.

Having demonstrated that saccadic inhibition occurs in
double-step tasks, Experiment 3 is designed to ask
whether inhibition plays a functional role in reorienting
behavior by testing the counterintuitive idea that re-
orienting behavior might actually benefit from additional
bottom–up interference, in the form a salient irrelevant
flash. Darüber hinaus, because Experiment 3 manipulates re-
flexive inhibition (the occurrence of flash) and voluntary
reorienting (the occurrence of a target step) without any
differences in task instruction, it provides a firmer basis
for assessing possible dissociations between the time
courses of inhibition and reorienting.

Experiment 3

Figur 6 shows the distributions for each participant,
with horizontal landing positions overlaid. Da ist ein

clear reduction in saccadic activity for all the conditions,
within a common temporal window. Yet we can also ob-
serve consistent differences between conditions, welche
are clear from the inhibition profiles in Figure 7 Und
the individual parameters in Table 1. The latency of inhi-
bition was lower in the flash and step-flash conditions
than in the step condition, T(5) = 6.27, p = .002; T(5) =
4.47, p = .007, and inhibition began earlier in these con-
ditions, T(5) = 4.05, P < .01; t(5) = 3.48, p = .02. By con- trast, inhibition ended later in the step and step-flash conditions compared with the flash condition, t(5) = 7.57, p = .001; t(5) = 3.96, p = .01. These patterns strongly support the idea that the total inhibitory period during reorienting is a combination of an early bottom–up effect (driven by the salient visual change) and a sustained effect (driven by goal-related reorienting). The addition of the flash to the step manipulation boosts the early component, and the intention to reorient influences the later component. Because the addition of the flash shifts the start of inhibition to an earlier moment, we can examine how this impacts upon reorienting behavior. The first thing to note is that there is no corresponding shift in the Buonocore, Purokayastha, and McIntosh 2075 D o w n l o a d e d f r o m l l / / / / j t t f / i t . : / / h t t p : / D / o m w i n t o p a r d c e . d s f i r o l m v e h r c p h a d i i r r e . c c t . o m m / j e o d u c n o / c a n r a t r i t i c c l e e - p - d p d 2 f 9 / 1 2 2 9 / 2 1 0 2 6 / 8 2 1 0 9 6 5 8 3 / 2 1 9 8 7 8 o 6 c 8 n 5 _ 7 a / _ j 0 o 1 c 1 n 7 9 _ a p _ d 0 1 b 1 y 7 g 9 u . e p s t d o f n b 0 y 8 S M e I p T e m L i b b e r r a 2 r 0 2 i 3 e s / j f / t . u s e r o n 1 7 M a y 2 0 2 1 Figure 5. Group average time course of proportional inhibition derived from the saccadic inhibition analysis for distractor (red) and step (blue) conditions of Experiment 2. For the step condition, the black line represents the group average direction transition function (right y axis). Time on the x axis is coded relative to transient onset (i.e., positive numbers indicate saccades launched after distractor onset), rather than initial target onset, to reveal the time course of responses to the transient change. direction transition function (average function in Figure 7; individual parameters in Table 2): The time course of reorienting is invariant across the shift in inhibition onset. Table 2 similarly shows that the median latency of reorienting to Target 2 was not reduced by the flash (if anything, there was a tendency toward an in- crease). This implies that saccadic inhibition does not facilitate faster reorienting to Target 2, but this does not mean that it has no functional benefit for reorienting behavior. The most crucial data are the probabilities of Target 1 responses in step and step-flash conditions (i.e., the error rate), reflecting the overall proportion of saccades in which reorienting was unsuccessful. The probability of Target 1 responses is relatively high in the step condition (mean = 0.41, SD = 0.24), but significantly reduced in the step-flash condition (mean probability of Target 1 = 0.31, SD = 0.21), a difference that was seen in every par- ticipant, t(5) = 6.78, p = .001. Reorienting thus becomes more reliable when inhibition is boosted by the flash. We infer that reflexive inhibition rapidly interrupts ongoing activity, allowing a higher proportion of saccades to be re- programmed for the new target, yet without speeding their redirection. DISCUSSION Our experiments show that saccadic inhibition occurs in typical double-step tasks and that it promotes successful reorienting. In our critical third experiment, double-step reorienting was improved by a distracting flash, coinci- dent with the change in target position, even though the flash carried no spatial information about the target. However, although reoriented saccades were more likely after a flash, we did not find that they took any less time to emerge. Thus, oculomotor inhibition improves re- orienting, but the benefit is quite specific: Inhibition helps to countermand the planned response at short latency, making an alternative response possible, but it does not speed the generation of that alternative response. The short-latency inhibitory effect of a visual change is apparent across all conditions of our three experiments. The onset of inhibition is insensitive to the task relevance of a visual change but is modulated by its salience. In Experiment 3, the onset of inhibition was thus earlier in conditions with a salient flash (flash and step-flash con- ditions) than in the simple target step condition, consis- tent with a low-level reflexive response to the flash impacting on a time scale close to the minimum neural delays for the visual information to reach the superior colliculus (Rizzolatti, Buchtel, Camarda, & Scandolara, 1980). By contrast, the late, recovery portion of the inhi- bition profile was affected by the task context, with the offset of inhibition relatively later in conditions requiring reorienting, presumably because it takes longer to pro- gram a new saccade than to restore a prior plan. So, re- flexive inhibition facilitates reorienting by automatically suppressing a planned saccade, but the generation of a replacement response depends on participant intentions and unfolds over a longer time scale. 2076 Journal of Cognitive Neuroscience Volume 29, Number 12 D o w n l o a d e d f r o m l l / / / / j f / t t i t . : / / h t t p : / D / o m w i n t o p a r d c e . d s f i r o l m v e h r c p h a d i i r r e . c c t . o m m / j e o d u c n o / c a n r a t r i t i c c l e e - p - d p d 2 f 9 / 1 2 2 9 / 2 1 0 2 6 / 8 2 1 0 9 6 5 8 3 / 2 1 9 8 7 8 o 6 c 8 n 5 _ 7 a / _ j 0 o 1 c 1 n 7 9 _ a p _ d 0 1 b 1 y 7 g 9 u . e p s t d o f n b 0 y 8 S M e I p T e m L i b b e r r a 2 r 0 2 i 3 e s / j f / t . u s e r o n 1 7 M a y 2 0 2 1 Figure 6. Each panel shows the probability density estimate derived from the raw data of each participant (P1–P6) in the flash (red), step (blue), and step-flash (green) conditions of Experiment 3. The horizontal components of the saccade landing positions are overlaid. Time on the x axis is coded relative to transient onset, rather than initial target onset, to reveal the time course of responses to the transient change. These findings are consistent with a contemporary model of double-step behavior, which suggests that re- orienting cannot succeed simply by activation of a re- placement response but also requires a STOP process to countermand the initial planned response (Camalier et al., 2007; see also Bissett & Logan, 2013). This “inde- pendent horse race” model involves three processes: a GO process (GO1) accumulating activation to respond to the first target; a STOP process, triggered by the target step, accumulating activation to countermand the GO1 response; and a second GO process (GO2), also trig- gered by the target step, accumulating activation to re- spond to the new target. Within this model, each process develops independently of the others, except that the STOP process inhibits the GO1 process if it reaches threshold first. Note that this mutual indepen- dence implies that a GO2 response will not be speeded by a successful STOP process; it will just be more likely to occur because GO1 has been withdrawn from the race. Applying this model to the present context, saccadic inhibition would be a rapidly rising STOP process, trig- gered by the target step, which countermands the initially planned (GO1) response whenever it reaches threshold first; on these occasions, the GO2 response will sub- sequently complete, and a reorienting saccade will follow. In our Experiment 3, the addition of a large flash, simul- taneous with the target step, would selectively boost the STOP signal. This would raise the likelihood that the STOP process achieves threshold before GO1, reducing the frequency of Target 1 saccades and thereby increas- ing the frequency of Target 2 saccades, yet without speeding them ( just as we observed). We can similarly apply the model to the differences between Experiments 1 and 2, in which all stimulus events were matched, except that the initial target persisted after the step in Experiment 2. This selective boosting of the GO1 process in Experiment 2 would make it less likely to be countermanded by STOP and thereby less likely to be superseded by GO2, so that inhibition would be reduced and reorienting less success- ful (again as we observed). Saccadic inhibition seems like a phenomenon ready-made for the role of STOP process in this independent race model. But, although functional independence of STOP and GO processes may work within computational models of double-step behavior, mutual independence seems less plausible at the neurophysiological level. Reingold and Buonocore, Purokayastha, and McIntosh 2077 D o w n l o a d e d f r o m l l / / / / j f / t t i t . : / / h t t p : / D / o m w i n t o p a r d c e . d s f i r o l m v e h r c p h a d i i r r e . c c t . o m m / j e o d u c n o / c a n r a t r i t i c c l e e - p - d p d 2 f 9 / 1 2 2 9 / 2 1 0 2 6 / 8 2 1 0 9 6 5 8 3 / 2 1 9 8 7 8 o 6 c 8 n 5 _ 7 a / _ j 0 o 1 c 1 n 7 9 _ a p _ d 0 1 b 1 y 7 g 9 u . e p s t d o f n b 0 y 8 S M e I p T e m L i b b e r r a 2 r 0 2 i 3 e s / j t / . f u s e r o n 1 7 M a y 2 0 2 1 Figure 7. Group average time course of proportional inhibition derived from the saccadic inhibition analysis for flash (red), step (blue), and step- flash (green) conditions of Experiment 3. In the step and step-flash conditions, the black lines represent the group average direction transition function (right y axis). Time on the x axis is coded relative to transient onset, rather than initial target onset, to reveal the time course of responses to the transient change. Stampe (2002) originally speculated that saccadic inhibi- tion might arise from competitive interactions between populations of neurons, within the motor maps of the intermediate superior colliculus, building up activity for saccades to target and distractor locations (Olivier, Dorris, & Munoz, 1999). Dorris, Olivier, and Munoz (2007) sub- sequently confirmed that preparatory activity for an ex- pected saccade target is decreased transiently by the onset of a distractor elsewhere in the visual field (nearby distractors can conversely facilitate build-up activity). Recently, a physiologically inspired model, incorporating long-range inhibition and local facilitation, has been found to simulate empirical patterns of saccadic inhibition with impressive accuracy (Bompas & Sumner, 2011, 2015). Superior colliculus neurons thus show competitive inter- actions sufficient for causal involvement in saccade gener- ation and countermanding (see also Paré & Hanes, 2003), though extrinsic connections may also be involved, partic- ularly with FEF (Brown, Hanes, Schall, & Stuphorn, 2008; Hanes et al., 1998) and BG (Schmidt, Leventhal, Mallet, Chen, & Berke, 2013; Hikosaka, Takikawa, & Kawagoe, 2000). In addition, a more direct inhibitory effect may in- volve omnipause neurons in the brainstem, which gate the activity of saccade burst neurons and show spikes of activity time-locked to sudden visual changes (Missal & Keller, 2002; Everling, Paré, Dorris, & Munoz, 1998). This signal might delay the execution of a planned saccade, creating a pause in behavior, during which competitive interactions between target and distractor activity would have time to play out. The saccadic inhibition phenome- non might then result from a combination of competitive integration within the superior colliculus and saccade gating in the brainstem followed by top–down signals from cortical areas such as the FEF (Peel, Hafed, Dash, Lomber, & Corneil, 2016), promoting either reorienting or a reinstatement of the original plan. How can we reconcile evidence for neural interactions among oculomotor STOP and GO signals, with an inde- pendent race model of double-step behavior? This “neural paradox” has been noted already within the response in- hibition literature, and to address it, an interactive race model was proposed, which allows interactions between STOP and GO processes (Boucher, Palmeri, Logan, & Schall, 2007; Boucher, Stuphorn, Logan, Schall, & Palmeri, 2007). This interactive model produces behavior equiva- lent to that of the independent version, provided that the influence of the STOP process is brief and potent (Boucher, Palmeri, et al., 2007; see also Salinas & Stanford, 2013). This would be consistent with the character of saccadic inhi- bition, most of the latency of which is accounted for by afferent delays, implying a brief and potent inhibitory inter- action. Frameworks thus exist for modeling response inhi- bition and response selection that allow underlying neural interactions while giving the appearance of independent processes at the behavioral level. Conclusions Saccadic inhibition is a reflexive phenomenon that fol- lows whenever a visual change occurs while a saccade is planned, even if the change is known to be irrelevant (e.g., Buonocore & McIntosh, 2008; Reingold & Stampe, 2002). We suggest that it plays a crucial role in behavior, acting as a bottom–up brake that buys the time needed for voluntary processes to act. Reflexive inhibition follows a visual change at latencies too brief for voluntary control; the voluntary part is whether to subsequently recover the original plan (distractor context) or to switch to an alter- native (double-step context). In a rapid behavioral system like the ocuolomotor one, saccadic inhibition may be a ubiquitous pause signal that provides the flexibility for voluntary behavior to emerge. 2078 Journal of Cognitive Neuroscience Volume 29, Number 12 Acknowledgments The authors are grateful to Eyal Reingold for discussion of the ideas behind these experiments and to Ziad Hafed and Antje Nuthmann for comments on an earlier draft of this paper. A. B. and R. D. M. designed the experiments. A. B. and S. P. carried out data collection. A. B. and R. D. M. analyzed and interpreted the data, and drafted the paper. All authors approved the final version of the paper for submission. 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