Comparing the cerebral hemispheres on the speed of spatial shifts of visual attention: evidence from serial search

Visual field asymmetries in numerosity processing

A small number of objects can be rapidly and accurately enumerated, whereas a larger number of objects can only be approximately enumerated. These subitizing and estimation abilities, respectively, are both spatial processes relying on extracting information across spatial locations. Nevertheless, whether and how these processes vary across visual field locations remains unknown. Here, we examined if enumeration displays asymmetries around the visual field. Experiment 1 tested small number (1–6) enumeration at cardinal and non-cardinal peripheral locations while manipulating the spacing among the objects. Experiment 2 examined enumeration at cardinal locations in more detail while minimising crowding. Both experiments demonstrated a Horizontal-Vertical Asymmetry (HVA) where performance was better along the horizontal axis relative to the vertical. Experiment 1 found that this effect was modulated by spacing with stronger asymmetry at closer spacing. Experiment 2 revealed further asymmetries: a Vertical Meridian Asymmetry (VMA) with better enumeration on the lower vertical meridian than on the upper and a Horizontal Meridian Asymmetry (HMA) with better enumeration along the left horizontal meridian than along the right. All three asymmetries were evident for both subitizing and estimation. HVA and VMA have been observed in a range of visual tasks, indicating that they might be inherited from early visual constraints. However, HMA is observed primarily in mid-level tasks, often involving attention. These results suggest that while enumeration processes can be argued to inherit low-level visual constraints, the findings are, parsimoniously, consistent with visual attention playing a role in both subitizing and estimation.

Characterizing the in-out asymmetry in visual crowding

An object's processing is impaired by the presence of nearby clutter. Several distinct mechanisms, such as masking and visual crowding, are thought to contribute to such flanker-induced interference. It is therefore important to determine which mechanism is operational in any given situation. Previous studies have proposed that the in-out asymmetry (IOA), where a peripheral flanker interferes with the target more than a foveal flanker, is diagnostic of crowding. However, several studies have documented inconsistencies in the occurrence of this asymmetry, particularly at locations beyond the horizontal meridian, casting doubt on its ability to delineate crowding. In this study, to determine if IOA is diagnostic of crowding, we extensively charted its properties. We asked a relatively large set of participants (n = 38) to identify a briefly presented peripheral letter flanked by a single inward or outward letter at one of four locations. We also manipulated target location uncertainty and attentional allocation by blocking, randomizing or pre-cueing the target location. Using multilevel Bayesian regression analysis, we found robust IOA at all locations, although its strength was modulated by target location, location uncertainty, and attentional allocation. Our findings suggest that IOA can be an excellent marker of crowding, to the extent that it is not observed in other flanker-interference mechanisms, such as masking.

Independent resources for attentional tracking in the left and right visual hemifields

The ability to divide attention enables people to keep track of up to four independently moving objects. We now show that this tracking capacity is independently constrained in the left and right visual fields as if separate tracking systems were engaged, one in each field. Specifically, twice as many targets can be successfully tracked when they are divided between the left and right hemifields as when they are all presented within the same hemifield. This finding places broad constraints on the anatomy and mechanisms of attentive tracking, ruling out a single attentional focus, even one that moves quickly from target to target.

Visual field asymmetries in selective attention: Evidence from a modified search paradigm

The present study investigated visual field differences in selective attention. Five stimuli were briefly presented and subjects were asked to identify a predefined target. The target/distractor physical similarity varied systematically (low, medium or high) in order to encourage attentional resolving. Right/left hemifield differences were examined in Experiment 1, temporal/nasal hemifield differences in Experiment 2, and upper/lower hemifield differences in Experiment 3. Visual field differences were found only in Experiment 1 suggesting a left/right hemispheric asymmetry in selective attention. These asymmetries appear with increasing stimuli similarity, and suggest that each hemisphere gets involved when attentional selection cannot be carried out without the mode of information processing that characterizes that hemisphere. The absence of other hemifield asymmetries is not in favor of neither a subcortical, nor a specific superior occipito-parietal involvement in attentional resolving and selectivity.

The neural basis of perceptual learning

Perceptual learning is a lifelong process. We begin by encoding information about the basic structure of the natural world and continue to assimilate information about specific patterns with which we become familiar. The specificity of the learning suggests that all areas of the cerebral cortex are plastic and can represent various aspects of learned information. The neural substrate of perceptual learning relates to the nature of the neural code itself, including changes in cortical maps, in the temporal characteristics of neuronal responses, and in modulation of contextual influences. Top-down control of these representations suggests that learning involves an interaction between multiple cortical areas.

Parallel and serial search in two teenagers with lesions of the mesial parietal cortex

Visual-spatial attention was examined in two 14-year-olds who had undergone occipital-parietal craniotomies for removal of mesial parietal tumors, one in the right and one in the left hemisphere. Neither patient showed clinically significant visual neglect. They were administered two visual search tasks from Treisman and Souther [43] that make significantly different demands on visual spatial attention. In feature-present (parallel) search, they searched for the presence of a feature. In feature-absent (serial) search, they searched for its absence. Search rate was estimated from the slope of the function relating display size to response time. Both patients had flat slopes in feature-present search to target-present (TP) displays, indicating that they could conduct parallel search at the same rate as controls. Although the patient with the right-hemisphere lesion also had a flat slope to target-absent (TA) displays, the patient with the left-hemisphere lesion had a steep slope (30 ms/item) in this condition. In feature-absent search, the patients had equally slow search rates compared to controls, suggesting that the mesial parietal cortex is part of the network that mediates serial shifts of attention. Results support the distinction between detection of the target in parallel vs serial search and suggest that processes involved in TP and TA trials in parallel search are also dissociable.

The role of the parietal cortex in visual attention--hemispheric asymmetries and the effects of learning: a magnetic stimulation study

Our previous studies of the role of the parietal cortex in visual learning and attention showed that the right parietal cortex is required for normal performance on conjunction visual search tasks but that its role depends on whether subjects are naive or trained on the task. Here we extend these findings in two Experiments. Experiment 1 shows that magnetic stimulation of the left parietal cortex also impairs performance (measured as reaction time) on conjunction visual search tasks, but only when the target is present in the right (contralateral) visual field. Stimulation of the same region on a feature detection task speeds up performance significantly when the target is in the left (ipsilateral) visual field. Experiment 2 explores further the role of the right parietal cortex in learning conjunction search tasks. Stimulation of the right parietal cortex in subjects who had already trained on some visual search tasks did not impair performance on a novel motion/form conjunction task even though the search was clearly serial. Stimulation of area V5, however, severely disrupted performance on the same task. These data indicate that the role of the parietal cortex may change much earlier in the course of training than initially thought.

Cortical plasticity in perceptual learning demonstrated by transcranial magnetic stimulation

Performance on a wide range of perceptual tasks improves with practice. Most accounts of perceptual learning are concerned with changes in neuronal sensitivity or changes in the way a stimulus is represented. Another possibility is that different areas of the brain are involved in performing a task while learning it and after learning it. Here we demonstrate that the right parietal cortex is involved in novel but not learned visual conjunction search. We observed that single pulse transcranial magnetic stimulation (TMS) to the right parietal cortex impairs visual conjunction search when the stimuli are novel and require a serial search strategy, but not once the particular search task has been learned. The effect of TMS returns when a different, novel, serial search task is presented.

Cortical plasticity in perceptual learning demonstrated by transcranial magnetic stimulation

Performance on a wide range of perceptual tasks improves with practice. Most accounts of perceptual learning are concerned with changes in neuronal sensitivity or changes in the way a stimulus is represented. Another possibility is that different areas of the brain are involved in performing a task during and after learning it. Here, we demonstrate that the right parietal cortex is involved in novel but not learned visual conjunction search. We observed that single pulse transcranial magnetic stimulation (TMS) to the right parietal cortex impairs visual conjunction search when the stimuli are novel and require a serial search strategy, but not once the particular search task has been learned. The effect of TMS returns when a different, novel, serial search task is presented.

Electrophysiologic correlates of visuo-spatial attention shift

We investigated neurophysiologic correlates of shifting visual attention across the visual field. Event-related potentials (ERPs) were recorded in 12 normal subjects during a visual discrimination task in which target stimuli were presented at a predictable or unpredictable location in the eccentric visual field. Subjects were obliged to shift their attention from the expected site to the unpredictable site immediately after the presentation of shifted stimuli in order to detect the change of stimulus attributes. Shifted stimuli modulated the N1 component (130-200 msec), producing a larger amplitude at the posterior temporal site contralateral to the stimulus field and a smaller amplitude over the ipsilateral hemisphere. Furthermore, shifted stimuli uniquely evoked a positive ERP component with a latency of 200-300 msec, which distributed broadly over the skull maximally at the frontal and central electrode sites. Both the negative and positive components changed in amplitude as a function of shift distance and direction. These results suggest that modulations of the negative and positive deflections reflect the shift of covert visuo-spatial attention and that right hemispheric dominance does not exist at least in the early stage of shifting spatial attention.

Illusory conjunctions and the cerebral hemispheres

Two visual half-field experiments tested Moscovitch's (1979) proposition that cerebral asymmetry does not concern the earliest perceptual stages but only later processing. Subjects were briefly shown displays that included one (Experiment 1) or two (Experiment 2) types of forms differing in size and which, according to previous evidence, might lead to opposite laterality effects. Laterality effects were assessed for correct detections and for illusory conjunctions, both in terms of raw detection scores and in terms of perceptual discriminability (d' scores). In Experiment 1, displays included either rectangles or triangles. In the first case, the target was a cross; in the second case, it was a Star of David. A hemifield x size interaction was observed both on correct detections and on associated discriminability. Yet, no such interaction was obtained for illusory conjunctions or for associated d' scores. In Experiment 2, the two types of forms were presented simultaneously, with the small ones either inside or outside the large ones. No laterality effects were observed. Some implications of these data for both hemispheric asymmetry and feature integration issues are discussed. The results suggest that early preattentive processes of feature extraction are not lateralized, whereas some integrative mechanisms, such as Treisman's (1988) focal attention, may operate differently in the two hemispheres.

Single-character processing in a case of pure alexia

The processing of single characters in a pure alexic patient was studied in an attempt to identify the impairment responsible for his reading disorder. Observations from Experiments 1 to 4 suggested a deficit of identification of alphanumeric stimuli without any impairment affecting the elaboration of a structural description of visual stimulation. Experiment 5 indicated that the identification disorder results from a defect in the selective processes--activation and/or inhibition--that must come into play for achieving an appropriate match between a structural description of the stimulation and representations of the identities of known stimuli. The possible implications of this deficit in single-character identification for word reading are discussed.

On the relation between visual spatial attention and visual field asymmetries

In the typical visual laterality experiment, words and letters are more rapidly and accurately identified in the right visual field than in the left. However, while such studies usually control fixation, the deployment of visual attention is rarely restricted. The present studies investigated the influence of visual attention on the visual field asymmetries normally observed in single-letter identification and lexical decision tasks. Attention was controlled using a peripheral cue that provided advance knowledge of the location of the forthcoming stimulus. The time period between the onset of the cue and the onset of the stimulus (Stimulus Onset Asynchrony--SOA) was varied, such that the time available for attention to focus upon the location was controlled. At short SOAs a right visual field advantage for identifying single letters and for making lexical decisions was apparent. However, at longer SOAs letters and words presented in the two visual fields were identified equally well. It is concluded that visual field advantages arise from an interaction of attentional and structural factors and that the attentional component in visual field asymmetries must be controlled in order to approximate more closely a true assessment of the relative functional capabilities of the right and left cerebral hemispheres.