Intertrial effects of randomization on saccadic reaction times in human observers

The influence of attention and target identification on saccadic eye movements depends on prior target location

Saccadic latency is reduced by a temporal gap between fixation point and target, by identification of a target feature, and by movement in a new direction (inhibition of saccadic return, ISR). A simple additive model was compared with a shared resources model that predicts a three-way interaction. Twenty naïve participants made horizontal saccades to targets left and right of fixation in a randomised block design. There was a significant three-way interaction among the factors on saccade latency. This was revealed in a two-way interaction between feature identification and the gap versus no gap factor which was only apparent when the saccade was in the same direction as the previous saccade. No interaction was apparent when the saccade was in the opposite direction. This result supports an attentional inhibitory effect that is present during ISR to a previous location which is only partly released by the facilitative effect of feature identification and gap. Together, anticipatory error data and saccade latency interactions suggest a source of ISR at a higher level of attention, possibly localised in the dorsolateral prefrontal cortex and involving tonic activation.

Deficit of inhibition motor control in untreated patients with schizophrenia: further support from visually guided saccade paradigms

In addition to classical delusional, negative, and cognitive deficit, schizophrenia has consistently been associated with impairments in saccadic eye movements, e.g., an increased error rate in the antisaccade task. We hypothesized that a deficit in inhibitory control is a core defect in untreated patients with schizophrenia leading to impairment in different oculomotor paradigms. Ten drug-free or drug-naïve patients with schizophrenia were matched in age and gender to 11 healthy controls with no psychoactive substance use or abuse. They were explored using reflexive saccades with unpredictable targets with or without the gap procedure, predictive saccades and a fixation/distracter paradigm. Patients with schizophrenia displayed shorter latency in reflexive and predictive saccades. In the GAP condition, patients made more anticipatory saccades, fewer regular saccades, and had a shorter latency of express saccades than controls. In addition, patients had an increased error rate in the fixation/distracters task. Altogether, these results provide new evidence of reduced prefrontal inhibitory regulation of subcortical and brainstem systems involved in the control of saccades.

The influence of motor training on human express saccade production

Express saccadic eye movements are saccades of extremely short latency. In monkey, express saccades have been shown to occur much more frequently when the monkey has been trained to make saccades in a particular direction to targets that appear in predictable locations. Such results suggest that express saccades occur in large number only under highly specific conditions, leading to the view that vector-specific training and motor preparatory processes are required to make an express saccade of a particular magnitude and direction. To evaluate this hypothesis in humans, we trained subjects to make saccades quickly to particular locations and then examined whether the frequency of express saccades depended on training and the number of possible target locations. Training significantly decreased saccade latency and increased express saccade production to both trained and untrained locations. Increasing the number of possible target locations (two vs. eight possible targets) led to only a modest increase of saccade latency. For most subjects, the probability of express saccade occurrence was much higher than that expected if vector-specific movement preparation were necessary for their production. These results suggest that vector-specific motor preparation and vector-specific saccade training are not necessary for express saccade production in humans and that increases in express saccade production are due in part to a facilitation in fixation disengagement or else a general enhancement in the ability of the saccadic system to respond to suddenly appearing visual stimuli.

Reduced attentional engagement contributes to deficits in prefrontal inhibitory control in schizophrenia

BACKGROUND

Problems with the voluntary control of behavior, such as those leading to increased antisaccade errors, are accepted as evidence of prefrontal dysfunction in schizophrenia. We previously reported that speeded prosaccade responses, i.e., shorter response latencies for automatic shifts of attention to visual targets, were associated with higher antisaccade error rates in schizophrenia. This suggests that dysregulation of automatic attentional processes may contribute to disturbances in prefrontally mediated control of voluntary behavior.

METHODS

Twenty-four antipsychotic-naïve schizophrenia patients and 30 healthy individuals completed three tasks: a no-gap prosaccade task in which subjects shifted gaze toward a peripheral target that appeared coincident with the disappearance of a central fixation target and separate prosaccade and antisaccade tasks in which a temporal gap or overlap of the central target offset and peripheral target onset occurred. Sixteen patients were retested after 6 weeks of antipsychotic treatment.

RESULTS

Patients' prosaccade latencies in the no-gap task were speeded compared with healthy individuals. While patients were not atypical in the degree to which response latencies were speeded or slowed by the gap and overlap manipulations, those patients with diminished attentional engagement on the prosaccade task (i.e., reduced overlap effect) had significantly elevated antisaccade error rates. This effect persisted in patients evaluated after antipsychotic treatment.

CONCLUSIONS

This study provides evidence that a reduced ability to engage attention may render patients more distracted by sensory inputs, thereby further compromising impaired executive control during antisaccade tasks. Thus, alterations in attentional and executive control functions can synergistically disrupt voluntary behavioral responses in schizophrenia.

Involuntary attention and identification accuracy

Using the spatial cuing paradigm, Prinzmetal, McCool, and Park (2005) made the distinction between voluntary and involuntary attention. They claimed that although accuracy was affected by an informative spatial cue (which controls voluntary attention), it was not affected by a noninformative cue (which controls involuntary attention). We reevaluate two reports that assert that noninformative spatial cues affect accuracy. Dufour (1999) reported that a noninformative auditory cue enhanced visual identification in a conjunction search task. Klein and Dick (2002) reported that, in an RSVP task with visual cues, the cue also enhanced accuracy at short stimulus onset asynchronies. We found that Dufour's results were due to overt orienting (eye movements) rather than to covert attention. The results of Klein and Dick were due either to location uncertainty or to a confounding of the order of stimulus presentation and condition.

What is the coordinate frame utilized for the generation of express saccades in monkeys?

The latencies of saccades to suddenly appearing eccentric targets can have a bimodal distribution, with an early, express peak, and a late, regular peak (Fischer and Boch 1983, Brain Res 260: 21-26). Express saccades usually are a product of learning. The purpose of this study was to determine whether this learning is specific to the relative position of the target in space, the orbital position of the eye, or the vector of the saccade to be produced. Further, it was asked whether and how the frequency with which express saccades are generated is influenced by the immediately preceding saccadic vector and the familiarity of the targets. To this end, rhesus monkeys were trained to make saccadic eye movements to single targets and to two sequential targets that appeared at various positions relative to the head, relative to the initial fixation spot and relative to each other. The results show that the frequency with which express saccades are generated is determined by the saccadic vector that has to be generated and not by the relative position of a target in space, the orbital position of the eye, the immediately preceding saccadic vector, or the familiarity of the targets.

Contextual influence of TMS on the latency of saccades and vergence

This study examines the effects of TMS of the right PPC on the latency of saccades and vergence alone or combined and the role of experimental design. Two designs were used: pure blocks with exclusively no-TMS or TMS trials; mixed blocks in which no-TMS and TMS trials were interleaved; a control study with TMS of the primary motor cortex (pure blocks) was also conducted and showed no effects on latencies. In contrast, in the experiment with TMS of the PPC latencies for TMS trials increased relative to no-TMS trials for almost all eye movements (isolated saccades, convergence, divergence, and for saccade and divergence components of combined eye movements). However, such increase was significant for pure blocks only. In mixed blocks no difference between TMS and no-TMS was found mainly because the latency of no-TMS trials increased relative to corresponding latencies in pure blocks. A second study centered on isolated convergence and divergence confirmed the interaction between block-design and TMS effects, and showed significant TMS/no-TMS differences only for the pure design and for a design in which the rate of TMS trials was high (75%). Again, the absence of difference was due to increase of latency for no-TMS trials in mixed blocks with low rates of TMS trials (50% or 25%), but also to decreased effects for the TMS trials themselves. We conclude that latency of all eye movements, saccades and vergence is highly influenced by the context. Such a contextual factor is the number of TMS versus no-TMS trials within a block; low numbers of TMS trials (50% or less) increases baseline latencies. The design of mixed blocks with 50% or less of TMS trials should not be recommended as it underestimates the direct effects of TMS on cortical processing. In fact, the majority of TMS studies on eye movements do use paradigms with high rates of TMS trials (75% or more). Our study confirms the validity of such paradigms.

Looking for the GAP effect in manual responses and the role of contextual influences in reaction time experiments

When the offset of a visual stimulus (GAP condition) precedes the onset of a target, saccadic reaction times are reduced in relation to the condition with no offset (overlap condition) - the GAP effect. However, the existence of the GAP effect for manual responses is still controversial. In two experiments using both simple (Experiment 1, N = 18) and choice key-press procedures (Experiment 2, N = 12), we looked for the GAP effect in manual responses and investigated possible contextual influences on it. Participants were asked to respond to the imperative stimulus that would occur under different experimental contexts, created by varying the array of warning-stimulus intervals (0, 300 and 1000 ms) and conditions (GAP and overlap): i) intervals and conditions were randomized throughout the experiment; ii) conditions were run in different blocks and intervals were randomized; iii) intervals were run in different blocks and conditions were randomized. Our data showed that no GAP effect was obtained for any manipulation. The predictability of stimulus occurrence produced the strongest influence on response latencies. In Experiment 1, simple manual responses were shorter when the intervals were blocked (247 ms, P < 0.001) in relation to the other two contexts (274 and 279 ms). Despite the use of choice key-press procedures, Experiment 2 produced a similar pattern of results. A discussion addressing the critical conditions to obtain the GAP effect for distinct motor responses is presented. In short, our data stress the relevance of the temporal allocation of attention for behavioral performance.

Responses to targets in the visual periphery in deaf and normal-hearing adults

The purpose of this study was to compare the response times of deaf and normal-hearing individuals to the onset of target events in the visual periphery in distracting and nondistracting conditions. Visual reaction times to peripheral targets placed at 3 eccentricities to the left and right of a center fixation point were measured in prelingually deafened adults and normal-hearing adults. Deaf participants responded more slowly than normal-hearing participants to targets in the near periphery in the nondistracting condition and to targets in the near and distant periphery when distracting stimuli were present. One interpretation of these findings is that deaf individuals may be more deliberate than normal-hearing individuals in responding to near peripheral events and to peripheral events that occur in the presence of distracting stimuli.

Automatic orienting of visuospatial attention in Parkinson's disease

The basal ganglia are involved in not only motor behavior, but also other more cognitive processes, such as attention. We tested Parkinson's disease (PD) patients in a task that measures reflexive orienting of spatial attention. Seven patients with idiopathic PD and eight control subjects performed a covert orienting task where spatial attention was directed by means of exogenous cues (luminance increments) with no predictive validity for target position. The subjects' task was to make a speeded saccade to a visual target, which appeared a variable time after onset of the cue either in the cued or an uncued spatial position. There was no overall difference between PD patients and control subjects in terms of the initial facilitation following reflexive cues, and later inhibition of return (IOR). However, PD patients differed from control subjects in two important respects. First, they were significantly faster than were control subjects on this reflexive visual-orienting task. Second, disease severity correlated with attentional performance; more advanced patients showed less initial facilitation but greater IOR. Thus PD patients show better performance on a reflexive saccade task and, for more advanced patients, greater IOR than control subjects. These findings are consistent with the possibility that reflexive attentional processes in PD patients may be more active.

Control' of reflexive and voluntary saccades in the gap effect

In two experiments, we examined whether voluntary and reflexive saccades shared a common fixation disengagement mechanism. Participants were required to perform a variety of tasks, each requiring a different level of information processing of the display prior to execution of the saccade. In Experiment 1, participants executed either a prosaccade or an antisaccade upon detecting a stimulus array. In Experiment 2, participants executed a prosaccade to a stimulus array only if the array contained a target item. The target could be a line (easy search) or a digit (difficult search). The critical manipulation in both experiments was the relative timing between the removal of the fixation stimulus and the onset of the stimulus array. In both experiments, it was found that saccadic latencies were shortest when the fixation stimulus was removed before the onset of the stimulus array--a gap effect. It was concluded that reflexive and voluntary saccades share a common fixation disengagement mechanism that is largely independent of higher level cognitive processes.

The gap effect during visual and auditory stimulation using manual responses

Six experiments were conducted in naive human participants to examine any facilitation produced in manual reaction time (RT) by the interposition of a temporal gap between a warning signal and an imperative signal. Peripheral visual stimuli and monoaural auditory stimuli were used as targets. Participants showed a facilitation of RTs to the targets for both auditory and visual stimuli in the five experiments in which RTs were the dependent variable. In addition, the gap effect increased over successive blocks of trials, suggesting learning. RTs were facilitated only when the gap had predictive value and was salient. Using a variable temporal gap or visual warning stimulus did not change the facilitation in RTs. A further experiment demonstrated that the gap can be perceived by the participants. The dissociation between a learned and a non-learned component in the gap effect suggests that the temporal gap induces two independent processes: warning and disengagement of attention.

Saccadic reaction times: a statistical analysis of multimodal distributions

The distributions of saccadic reaction times (SRT) often deviate from unimodal normal distributions. An excess-mass procedure was used to detect peaks in 963 data sets containing 90,927 reaction times from 170 subjects. About 55% showed one, 30% two, 12% three and 3% four peaks. According to their clustering along the reaction time scale the modes could be classified into express (90-120 msec), fast regular (135-170 msec) and slow regular (200-220 msec) modes. Among the unimodal distributions 29% had peaks in the range of the express mode and 46% had peaks in the range of the fast regular mode. Therefore, 87% of the data sets support the notion of saccadic reaction time distributions being the superposition of three modes. All experimental distributions were fitted by as many gamma distributions as determined by the excess-mass test. The significance of the multimodality for saccade generation processes is discussed.

Presaccadic processes in the generation of pro and anti saccades in human subjects--a reaction-time study

It has been widely acknowledged that the generation of anti saccade, ie a saccade towards the direction opposite to that of a visual stimulus, requires the correct function of special brain structures. In the present study attempts were made to measure the time consumption of brain processes preceding the execution of pro and anti saccades. The saccadic eye movements of five adult human subjects were investigated in a series of combined pro/anti saccade tasks with the aid of the gap and the overlap paradigms. The type of trial--pro saccade and anti saccades--was defined by the structure of the stimulus. In some sessions the subjects were, in addition, preinformed about the actual command by a cue at different lead times before stimulus onset. Pro-saccade and anti-saccade trials were randomly intermixed in equal proportions. High error rates (> 30% of all trials in some subjects) occurred in the test sessions without preinformative cues. These errors had long reaction times (approximately 200 ms), whereas the latencies of correct pro or correct anti saccades were even longer (approximately 350 ms). Analysis of the errors revealed that they were related to the situation in the previous trial: a correct response in the previous trial enhanced the chance of making a saccade of the same type in the actual trial by up to 30%. This pretrial effect occurred whether the actual trial was a pro-saccade or an anti-saccade command. With a cue lead time of 100 ms the numbers of errors decreased, but the latencies of the correct pro or anti saccades were about 70 ms longer than those obtained in the nonrandom control. With a 200 ms cue lead time the reaction times corresponded to those in the control condition. The results suggest that the situation in a given trial creates a kind of default program for the saccade preparation in the next trial. When a cue about the actual command is given early enough, the default program is overridden correspondingly. The perception of the cue and the programming of the corresponding saccade take an additional 150 to 200 ms.