The Effects of the Fixation Cue in Inhibition of Return

In experiments examining inhibition of return (IOR), an attentional effect that inhibits the returning of attention to a previously attended location or object, a second cue during the stimulus onset asynchrony (SOA) period is typically used. This is done to control the reorienting of attention from a peripherally cued location back to the central fixation point before the target appears. Recently, there have been numerous studies which demonstrate that fixation cues are effective in revealing IOR. Plenty of factors have been shown to influence the effects of the fixation cue in IOR, including the time onset of the fixation cue, the number of potential target locations, the attentional demands of performing the task, the modality of the fixation cue, and the condition of participants. Here, the authors review previous work that has examined the effects of the fixation cue in IOR.

Altered Control of Visual Fixation and Saccadic Eye Movements in Attention-Deficit Hyperactivity Disorder

Attention-deficit hyperactivity disorder (ADHD) is characterized by the overt symptoms of impulsiveness, hyperactivity, and inattention. A frontostriatal pathophysiology has been hypothesized to produce these symptoms and lead to reduced ability to inhibit unnecessary or inappropriate behavioral responses. Oculomotor tasks can be designed to probe the ability of subjects to generate or inhibit reflexive and voluntary responses. Because regions of the frontal cortex and basal ganglia have been identified in the control of voluntary responses and saccadic suppression, we hypothesized that children and adults diagnosed with ADHD may have specific difficulties in oculomotor tasks requiring the suppression of reflexive or unwanted saccadic eye movements. To test this hypothesis, we measured eye movement performance in pro- and anti-saccade tasks of 114 ADHD and 180 control participants ranging in age from 6 to 59 yr. In the pro-saccade task, participants were instructed to look from a central fixation point toward an eccentric visual target. In the anti-saccade task, stimulus presentation was identical, but participants were instructed to suppress the saccade to the stimulus and instead look from the central fixation point to the side opposite the target. The state of fixation was manipulated by presenting the target either when the central fixation point was illuminated (overlap condition) or at some time after it disappeared (gap condition). In the pro-saccade task, ADHD participants had longer reaction times, greater intra-subject variance, and their saccades had reduced peak velocities and increased durations. In the anti-saccade task, ADHD participants had greater difficulty suppressing reflexive pro-saccades toward the eccentric target, increased reaction times for correct anti-saccades, and greater intra-subject variance. In a third task requiring prolonged fixation, ADHD participants generated more intrusive saccades during periods when they were required to maintain steady fixation. The results suggest that ADHD participants have reduced ability to suppress unwanted saccades and control their fixation behavior voluntarily, a finding that is consistent with a fronto-striatal pathophysiology. The findings are discussed in the context of recent neurophysiological data from nonhuman primates that have identified important control signals for saccade suppression that emanate from frontostriatal circuits.

Central Fixations are Inadequately Controlled by Instructions Alone: Implications for Studying Cerebral Asymmetry

A fundamental concern when using visual presentations to study cerebral asymmetry is to ensure that stimuli are presented with the same degree of retinal eccentricity from a central fixation point in either visual field. However, a widely used procedure intended to control fixation location merely instructs participants to fixate appropriately without any other means of ensuring that central fixations actually occur. We assessed the validity of assuming that instructions alone ensure central fixation by using the traditional R VF advantage for words and either (a) only instructions to fixate centrally, or (b) an eye-tracking device that ensured central fixation on every trial. Experiments 1 and 2 found that when only instructions were given, the vast majority of fixations were not central, and more occurred to the right of centre than to the left. Moreover, the prevalence of non-central fixations was otherwise disguised by the finding that both fixation procedures produced similar R VF advantages in overt performance. The impact of typical non-central fixations on performance was revealed by systematically manipulating fixation location in Experiment 3, where deviations in fixation of only 0.25° from centre had a reliable impact on visual field effects. Implications of these findings for studies of cerebral asymmetry are discussed.

Transforming Incompatible Stimulus-Response Pairs into Compatible Ones

The design of human-machine systems requires the consideration of a wealth of factors affecting human performance. One important factor concerns stimulus-response (S-R) compatibility effects. S-R compatibility refers to the finding that people respond faster when the relative spatial locations of stimuli and responses correspond (compatible S-R mappings) than when there is no correspondence (incompatible S-R mappings). For example, a visual stimulus that is presented to the right of a central fixation point is responded to more quickly by pressing a right key rather than a left key. In two experiments participants responded to compatible and incompatible S-R mappings. In Experiment 1 the stimuli were perceptually grouped while in Experiment 2 they were arbitrarily associated. Both perceptual grouping and arbitrary associations were found to result in the transformation of incompatible S-R pairs into compatible ones. The results are of practical use for tasks requiring human-machine interactions.

Effects of superior temporal polysensory area lesions on eye movements in the macaque monkey

1. On the basis of its anatomic connections and single-unit properties, the superior temporal polysensory area (STP) would seem to be primarily involved in visuospatial functions. We have examined the effects of lesions of STP on saccadic eye movements, visual fixation, and smooth pursuit eye movements to directly test the hypothesis that STP is involved in visuospatial and visuomotor behavior. 2. Seven monkeys were trained to make saccades to targets 8, 15, and 22 degrees from a central fixation point along the horizontal meridian and 8 degrees from the central fixation point along the vertical meridian. One monkey was also trained to make saccades to auditory targets. The same monkeys were trained to foveate a stationary central fixation point and to follow it with a smooth pursuit eye movement when it began moving 5, 13, or 20 degrees/s. Four monkeys received unilateral STP lesions, one received a bilateral STP lesion, and as a control, two received unilateral inferior temporal cortex (IT) lesions. After testing, three of the animals with unilateral STP lesions received an additional STP lesion in the hemisphere contralateral to the first lesion. Similarly, one animal with a unilateral IT lesion received an additional IT lesion in the hemisphere contralateral to the first lesion. 3. All monkeys with complete removal of STP showed a significant increase in saccade latency to the most peripheral contralateral target, and most also had increased saccade latencies to the other contralateral targets. Saccades directed to targets along the vertical meridian or toward targets in the hemifield ipsilateral to the lesion were not impaired by removal of STP. By contrast, IT lesions did not impair the monkeys' ability to make saccadic eye movements to visual stimuli at any location, showing that saccades to visually guided targets are not impaired nonspecifically by damage to visual cortex. 4. The deficit in making eye movements after STP lesions was specific to saccade latency, with little effect on the accuracy of saccades to visual targets. 5. In the one monkey trained to make saccades to auditory targets, removal of STP did not impair saccades to auditory targets contralateral to its lesion, despite this monkey showing the largest increase in saccades latencies to visual targets. 6. There was complete recovery of saccade latency to the baseline level of performance on the saccade task after all STP lesions.(ABSTRACT TRUNCATED AT 400 WORDS)

Some Factors in the Recognition of Tachistoscopically Presented Alphabetical Arrays

Six-letter nonsense arrays, constructed from a 12-letter population which was not made known to 20 Ss, were tachistoscopically shown successively in the right visual field (RVF) and left visual field (LVF) at three displacements from a central fixation point. Recognition scores were higher for stimuli in the RVF at each displacement. In each case RVF superiority was limited to letters in the first array-half (letters 1, 2, 3). These results agree with prior findings obtained with targets made up of six letters known to S (Fudin, 1969). Reportability of a tachistoscopically exposed letter, in part, is a function of the relationship between its retinal location and the delay before it is scanned. Location determines the amount of information a letter contains before it starts to fade-out (the more acute the area, the greater the information), delay determines the period of information dissipation prior to scanning. These ideas were used to explain the contrast between high error scores for middle-array letters in the bow-shaped error curves found in this experiment and the low values often reported for these letters when targets are shown across fixation, and the finding that the difference between error scores for letters in the first and second array-halves was greater for targets at each displacement in the right than the left visual field.

Static Visual Noise and the Ansbacher Effect

In Experiment I subjects made estimates of the apparent length of a 12 cm. long light arc-line rotating around a central fixation point at arc speeds of 66, 99, 132 and 165 cm./sec. Apparent contraction of the arc occurred as a function of speed, there being greater contraction in the presence of a superimposed random pattern of I cm. light squares (visual noise) than in its absence. The results of Experiment II indicated that there was no significant difference in length due to visual noise when the arcs were stationary. In Experiment III subjects made estimates of the speed of rotation of the arc and it was found that visual noise did not significantly affect the apparent arc-speed.