Foveal Presentation Research Articles

Geometrically restricted image descriptors: A method to capture the appearance of shape.

Shape perception varies depending on many factors. For example, presenting a stimulus in the periphery often yields a different appearance compared with its foveal presentation. However, how exactly shape appearance is altered under different conditions remains elusive. One reason for this is that studies typically measure identification performance, leaving details about target appearance unknown. The lack of appearance-based methods and general challenges to quantify appearance complicate the investigation of shape appearance. Here, we introduce Geometrically Restricted Image Descriptors (GRIDs), a method to investigate the appearance of shapes. Stimuli in the GRID paradigm are shapes consisting of distinct line elements placed on a grid by connecting grid nodes. Each line is treated as a discrete target. Observers are asked to capture target appearance by placing lines on a freely viewed response grid. We used GRIDs to investigate the appearance of letters and letter-like shapes. Targets were presented at 10° eccentricity in the right visual field. Gaze-contingent stimulus presentation was used to prevent eye movements to the target. The data were analyzed by quantifying the differences between targets and response in regard to overall accuracy, element discriminability, and several distinct error types. Our results show how shape appearance can be captured by GRIDs, and how a fine-grained analysis of stimulus parts provides quantifications of appearance typically not available in standard measures of performance. We propose that GRIDs are an effective tool to investigate the appearance of shapes.

Attribute amnesia can be modulated by foveal presentation and the pre-allocation of endogenous spatial attention.

Even in sparse visual environments, observers may not be able to report features of objects they have just encountered in a surprise question. Attribute amnesia and seeing without knowing describe report failures for irrelevant features of objects that have been processed to some extent in the primary task. Both phenomena are attributed to the exclusive selection of relevant information for memory consolidation or for awareness, respectively. While attribute amnesia was found even for irrelevant attributes of the target in the primary task, seeing without knowing was not observed when a single object was presented foveally. To elucidate this discrepancy, we examined report failures for irrelevant attributes of single target objects, which were presented either in the fovea or in the periphery, and either at cued or uncued locations. On a surprise trial, observers were able to report the irrelevant shape and color of the target object when it was presented foveally. However, presenting the same object just slightly away from the fovea led to report failures for shape. Introducing a valid peripheral cue prior to target presentation reduced report failures for shape when the cue was predictive of the target location, suggesting that the pre-allocation of endogenous spatial attention promoted the processing of irrelevant shape information. In accordance with previous research, we suggest that these modulations are due to differences in late selection for conscious awareness or consolidation in working memory.

Calibration of peripheral perception of shape with and without saccadic eye movements.

The cortical representations of a visual object differ radically across saccades. Several studies claim that the visual system adapts the peripheral percept to better match the subsequent foveal view. Recently, Herwig, Weiß, and Schneider (2015, Annals of the New York Academy of Sciences, 1339(1), 97-105) found that the perception of shape demonstrates a saccade-dependent learning effect. Here, we ask whether this learning actually requires saccades. We replicated Herwig et al.'s (2015) study and introduced a fixation condition. In a learning phase, participants were exposed to objects whose shape systematically changed during a saccade, or during a displacement from peripheral to foveal vision (without a saccade). In a subsequent test, objects were perceived as less (more) curved if they previously changed from more circular (triangular) in the periphery to more triangular (circular) in the fovea. Importantly, this pattern was seen both with and without saccades. We then tested whether a variable delay between the presentations of the peripheral and foveal objects would affect their association-hypothetically weakening it at longer delays. Again, we found that shape judgments depended on the changes experienced during the learning phase and that they were similar in both the saccade and fixation conditions. Surprisingly, they were not affected by the delay between the peripheral and foveal presentations over the range we tested. These results suggest that a general associative process, independent of saccade execution, contributes to the perception of shape across viewpoints.

Numerosity estimation benefits from transsaccadic information integration.

Humans achieve a stable and homogeneous representation of their visual environment, although visual processing varies across the visual field. Here we investigated the circumstances under which peripheral and foveal information is integrated for numerosity estimation across saccades. We asked our participants to judge the number of black and white dots on a screen. Information was presented either in the periphery before a saccade, in the fovea after a saccade, or in both areas consecutively to measure transsaccadic integration. In contrast to previous findings, we found an underestimation of numerosity for foveal presentation and an overestimation for peripheral presentation. We used a maximum-likelihood model to predict accuracy and reliability in the transsaccadic condition based on peripheral and foveal values. We found near-optimal integration of peripheral and foveal information, consistently with previous findings about orientation integration. In three consecutive experiments, we disrupted object continuity between the peripheral and foveal presentations to probe the limits of transsaccadic integration. Even for global changes on our numerosity stimuli, no influence of object discontinuity was observed. Overall, our results suggest that transsaccadic integration is a robust mechanism that also works for complex visual features such as numerosity and is operative despite internal or external mismatches between foveal and peripheral information. Transsaccadic integration facilitates an accurate and reliable perception of our environment.

The Role of Visual Eccentricity on Preference for Abstract Symmetry.

This study tested preference for abstract patterns, comparing random patterns to a two-fold bilateral symmetry. Stimuli were presented at random locations in the periphery. Preference for bilateral symmetry has been extensively studied in central vision, but evaluation at different locations had not been systematically investigated. Patterns were presented for 200 ms within a large circular region. On each trial participant changed fixation and were instructed to select any location. Eccentricity values were calculated a posteriori as the distance between ocular coordinates at pattern onset and coordinates for the centre of the pattern. Experiment 1 consisted of two Tasks. In Task 1, participants detected pattern regularity as fast as possible. In Task 2 they evaluated their liking for the pattern on a Likert-scale. Results from Task 1 revealed that with our parameters eccentricity did not affect symmetry detection. However, in Task 2, eccentricity predicted more negative evaluation of symmetry, but not random patterns. In Experiment 2 participants were either presented with symmetry or random patterns. Regularity was task-irrelevant in this task. Participants discriminated the proportion of black/white dots within the pattern and then evaluated their liking for the pattern. Even when only one type of regularity was presented and regularity was task-irrelevant, preference evaluation for symmetry decreased with increasing eccentricity, whereas eccentricity did not affect the evaluation of random patterns. We conclude that symmetry appreciation is higher for foveal presentation in a way not fully accounted for by sensitivity.

Object Substitution Masking for an Attended and Foveated Target

A central assumption of models proposed to explain object substitution masking (OSM) is that the phenomenon arises only when attention is distributed across several possible target locations. However, recent work has questioned the role of attention in OSM, suggesting instead that ceiling effects might explain the apparent interaction between spatial attention and masking. Here the authors report definitive evidence that OSM does not depend upon attention being distributed over space or time. In 2 experiments, they demonstrate reliable OSM for constant, foveal presentations of a single target stimulus. Crucially, in their design participants’ attention was always focused on the target, thus discounting the hypothesis that a key requirement for OSM is distributed attention. The findings challenge how OSM is conceptualized in the broader masking literature and have important implications for theories of visual processing.

Locating the cortical bottleneck for slow reading in peripheral vision.

Yu, Legge, Park, Gage, and Chung (2010) suggested that the neural bottleneck for slow peripheral reading is located in nonretinotopic areas. We investigated the potential rate-limiting neural site for peripheral reading using fMRI, and contrasted peripheral reading with recognition of peripherally presented line drawings of common objects. We measured the BOLD responses to both text (three-letter words/nonwords) and line-drawing objects presented either in foveal or peripheral vision (10° lower right visual field) at three presentation rates (2, 4, and 8/second). The statistically significant interaction effect of visual field × presentation rate on the BOLD response for text but not for line drawings provides evidence for distinctive processing of peripheral text. This pattern of results was obtained in all five regions of interest (ROIs). At the early retinotopic cortical areas, the BOLD signal slightly increased with increasing presentation rate for foveal text, and remained fairly constant for peripheral text. In the Occipital Word-Responsive Area (OWRA), Visual Word Form Area (VWFA), and object sensitive areas (LO and PHA), the BOLD responses to text decreased with increasing presentation rate for peripheral but not foveal presentation. In contrast, there was no rate-dependent reduction in BOLD response for line-drawing objects in all the ROIs for either foveal or peripheral presentation. Only peripherally presented text showed a distinctive rate-dependence pattern. Although it is possible that the differentiation starts to emerge at the early retinotopic cortical representation, the neural bottleneck for slower reading of peripherally presented text may be a special property of peripheral text processing in object category selective cortex.

Object substitution masking for an attended and foveated target.

A central assumption of models proposed to explain object substitution masking (OSM) is that the phenomenon arises only when attention is distributed across several possible target locations. However, recent work has questioned the role of attention in OSM, suggesting instead that ceiling effects might explain the apparent interaction between spatial attention and masking. Here the authors report definitive evidence that OSM does not depend upon attention being distributed over space or time. In 2 experiments, they demonstrate reliable OSM for constant, foveal presentations of a single target stimulus. Crucially, in their design participants' attention was always focused on the target, thus discounting the hypothesis that a key requirement for OSM is distributed attention. The findings challenge how OSM is conceptualized in the broader masking literature and have important implications for theories of visual processing.

The effect of spatial frequency on peripheral collinear facilitation

The detection of a Gabor patch (target) can be decreased or improved by the presence of co-oriented Gabor patches (flankers) having the same spatial frequency as the target. These phenomena are thought to be mediated by lateral interactions. Depending on the distance between target and flankers, commonly defined as a multiple of the wavelength (λ) of the carrier, flankers can increase or decrease a target's detectability. Studies with foveal presentation showed that for target-to-flankers distances<2λ contrast thresholds for the central target increase, while for target-to-flankers distances>3λ contrast thresholds decrease. Earlier studies on collinear facilitation at the near-periphery of the visual field (4° of eccentricity) showed inconsistent facilitation (Shani & Sagi, 2005, Vision Research, 45, 2009-2024) whereas more recent studies showed consistent facilitation for larger separations (7-8λ) (Maniglia et al., 2011, PLoS ONE, 6, e25568; Lev & Polat, 2011, Vision Research, 51, 2488-2498). However, all of these studies used medium-to-high spatial frequencies (3-8 cpd). In this study we tested lower spatial frequencies (1, 2, and 3 cpd) with different target-to-flankers distances. The rationale was that near-peripheral vision is tuned for lower spatial frequencies and this could be reflected in collinear facilitation. Results show consistent collinear facilitation at 8λ for all the spatial frequencies tested, but also show collinear facilitation at shorter target-to-flanker distance (6λ) for the lowest spatial frequencies tested (1 cpd). Additionally, collinear facilitation decreases as spatial frequency increases; opposite to the findings of Polat (2009, Spatial Vision, 22, 179-193) in the fovea, indicating a different spatial frequency tuning between foveal and peripheral lateral interactions.

Is theta burst stimulation applied to visual cortex able to modulate peripheral visual acuity?

Repetitive transcranial magnetic stimulation is usually applied to visual cortex to explore the effects on cortical excitability. Most researchers therefore concentrate on changes of phosphene threshold, rarely on consequences for visual performance. Thus, we investigated peripheral visual acuity in the four quadrants of the visual field using Landolt C optotypes before and after repetitive stimulation of the visual cortex. We applied continuous and intermittend theta burst stimulation with various stimulation intensities (60%, 80%, 100%, 120% of individual phosphene threshold) as well as monophasic and biphasic 1 Hz stimulation, respectively. As an important result, no serious adverse effects were observed. In particular, no seizure was induced, even with theta burst stimulation applied with 120% of individual phosphene threshold. In only one case stimulation was ceased because the subject reported intolerable pain. Baseline visual acuity decreased over sessions, indicating a continuous training effect. Unexpectedly, none of the applied transcranial magnetic stimulation protocols had an effect on performance: no change in visual acuity was found in any of the four quadrants of the visual field. Binocular viewing as well as the use of peripheral instead of foveal presentation of the stimuli might have contributed to this result. Furthermore, intraindividual variability could have masked the TMS- induced effects on visual acuity.

Age-Related Sparing of Parafoveal Lexical Processing

Background/Study Context: This study examined the role of increased adult age in the processing of lexical information presented in foveal and parafoveal areas of the retina (right and left visual fields). Previous research has shown that older adults are able to compensate for age-related changes though a highly practiced skill (Salthouse, 1984, Journal of Experimental Psychology: General, 113, 345–371). The authors examined if older adults would show a lexical-only parafoveal benefit. Methods: Two experiments were conducted on both younger and older adults. In Experiment 1, participants completed a lexical decision task that presented words in the fovea and parafovea in both visual fields. In Experiment 2, the task was a font discrimination task (nonlexical) with foveal and parafoveal presentation in both visual fields. Results: In Experiment 1, the authors observed word frequency effects for both foveal and right parafoveal presentation locations. This effect was present for both older and younger adults. Experiment 2 was a font discrimination task and there was no right parafoveal advantage for older adults on this task, suggesting that this effect observed in Experiment 1 was lexical in nature due to the highly overlearned nature of word recognition. Conclusion: These results suggest that older adults may compensate for slower encoding time in reading by encoding text to the right of fixation more efficiently than younger adults. This suggests an asymmetrical change in the useful field of view that is lexical in nature.

Right fusiform response patterns reflect visual object identity rather than semantic similarity

We previously reported the neuropsychological consequences of a lesion confined to the middle and posterior part of the right fusiform gyrus (case JA) causing a partial loss of knowledge of visual attributes of concrete entities in the absence of category-selectivity (animate versus inanimate). We interpreted this in the context of a two-step model that distinguishes structural description knowledge from associative-semantic processing and implicated the lesioned area in the former process. To test this hypothesis in the intact brain, multi-voxel pattern analysis was used in a series of event-related fMRI studies in a total of 46 healthy subjects. We predicted that activity patterns in this region would be determined by the identity of rather than the conceptual similarity between concrete entities. In a prior behavioral experiment features were generated for each entity by more than 1000 subjects. Based on a hierarchical clustering analysis the entities were organised into 3 semantic clusters (musical instruments, vehicles, tools). Entities were presented as words or pictures. With foveal presentation of pictures, cosine similarity between fMRI response patterns in right fusiform cortex appeared to reflect both the identity of and the semantic similarity between the entities. No such effects were found for words in this region. The effect of object identity was invariant for location, scaling, orientation axis and color (grayscale versus color). It also persisted for different exemplars referring to a same concrete entity. The apparent semantic similarity effect however was not invariant. This study provides further support for a neurobiological distinction between structural description knowledge and processing of semantic relationships and confirms the role of right mid-posterior fusiform cortex in the former process, in accordance with previous lesion evidence.

Not one extrastriate body area: Using anatomical landmarks, hMT+, and visual field maps to parcellate limb-selective activations in human lateral occipitotemporal cortex

The prevailing view of human lateral occipitotemporal cortex (LOTC) organization suggests a single area selective for images of the human body (extrastriate body area, EBA) that highly overlaps with the human motion-selective complex (hMT+). Using functional magnetic resonance imaging with higher resolution (1.5mm voxels) than past studies (3-4mm voxels), we examined the fine-scale spatial organization of these activations relative to each other, as well as to visual field maps in LOTC. Rather than one contiguous EBA highly overlapping hMT+, results indicate three limb-selective activations organized in a crescent surrounding hMT+: (1) an activation posterior to hMT+ on the lateral occipital sulcus/middle occipital gyrus (LOS/MOG) overlapping the lower vertical meridian shared between visual field maps LO-2 and TO-1, (2) an activation anterior to hMT+ on the middle temporal gyrus (MTG) consistently overlapping the lower vertical meridian of TO-2 and extending outside presently defined visual field maps, and (3) an activation inferior to hMT+ on the inferotemporal gyrus (ITG) overlapping the parafoveal representation of the TO cluster. This crescent organization of limb-selective activations surrounding hMT+ is reproducible over a span of three years and is consistent across different image types used for localization. Further, these regions exhibit differential position properties: preference for contralateral image presentation decreases and preference for foveal presentation increases from the limb-selective LOS to the MTG. Finally, the relationship between limb-selective activations and visual field maps extends to the dorsal stream where a posterior IPS activation overlaps V7. Overall, our measurements demonstrate a series of LOTC limb-selective activations that 1) have separate anatomical and functional boundaries, 2) overlap distinct visual field maps, and 3) illustrate differential position properties. These findings indicate that category selectivity alone is an insufficient organization principle for defining brain areas. Instead, multiple properties are necessary in order to parcellate and understand the functional organization of high-level visual cortex.

Semantic categorization precedes affective evaluation of visual scenes.

We compared the primacy of affective versus semantic categorization by using forced-choice saccadic and manual response tasks. Participants viewed paired emotional and neutral scenes involving humans or animals flashed rapidly in extrafoveal vision. Participants were instructed to categorize the targets by saccading toward the location occupied by a predefined target scene. The affective task involved saccading toward an unpleasant or pleasant scene, and the semantic task involved saccading toward a scene containing an animal. Both affective and semantic target scenes could be reliably categorized in less than 220 ms, but semantic categorization was always faster than affective categorization. This finding was replicated with singly, foveally presented scenes and manual responses. In comparison with foveal presentation, extrafoveal presentation slowed down the categorization of affective targets more than that of semantic targets. Exposure threshold for accurate categorization was lower for semantic information than for affective information. Superordinate-, basic-, and subordinate-level semantic categorizations were faster than affective evaluation. We conclude that affective analysis of scenes cannot bypass object recognition. Rather, semantic categorization precedes and is required for affective evaluation.

Neural correlates of fovea-related impairment of visual object processing in amblyopia

Event Abstract Back to Event Neural correlates of fovea-related impairment of visual object processing in amblyopia Judit Kortvelyes1, 2*, E. Banko1, Viktor Gal3, 4, G. Papay1, P. Domsa1, Jozsef Nemeth2 and Zoltan Vidnyanszky1, 3, 4 1 Pazmany Peter Catholic University, Hungary 2 Department of Ophthalmology, Semmelweis University, Hungary 3 Magnetic Resonance Research Center, Szentagothai J. Knowledge Center , Semmelweis University, Hungary 4 Neurobionics Research Group, Pazmany Peter Catholic University – Semmelweis University, Hungary Recent findings suggest that object perception is impaired in amblyopia. We used event-related potentials (ERP) to investigate the neural correlates of object processing in amblyopia in the case of foveal and perifoveal presentation of visual objects. Amblyops (N=16) and healthy controls (N=18) participated in the study. Face and hand grayscale images were used as stimuli. Images were presented without adding noise or at three different noise levels. Observers performed a 2AFC face vs hand discrimination task. Neuronal activity was measured using EEG (64 electrodes). In the amblyops, object discrimination performance was significantly decreased when stimuli were presented to the amblyopic eye compared to the fellow eye only in the foveal, but not in the perifoveal presentation condition. In the case of foveal presentation, the amplitude of face-specific N170 component was strongly reduced and its latency was increased when stimuli were presented to the amblyopic eye as compared to the fellow eye. Furthermore, in the case of the amblyopic eye, noise-dependent modulation of P220 component was strongly reduced in the foveal presentation condition. These findings provide evidence that in amblyops foveal representation of visual objects is impaired at the higher, face-specific stages of visual processing and suggest that the P220 component represents an ERP marker of inefficient neural processing of visual objects in the presence of noise in amblyopia. Conference: 12th Meeting of the Hungarian Neuroscience Society, Budapest, Hungary, 22 Jan - 24 Jan, 2009. Presentation Type: Poster Presentation Topic: Research on the cerebral cortex and related structures Citation: Kortvelyes J, Banko E, Gal V, Papay G, Domsa P, Nemeth J and Vidnyanszky Z (2009). Neural correlates of fovea-related impairment of visual object processing in amblyopia. Front. Syst. Neurosci. Conference Abstract: 12th Meeting of the Hungarian Neuroscience Society. doi: 10.3389/conf.neuro.01.2009.04.208 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 06 Mar 2009; Published Online: 06 Mar 2009. * Correspondence: Judit Kortvelyes, Pazmany Peter Catholic University, Budapest, Hungary, kortvelyes.judit@itk.ppke.hu Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Judit Kortvelyes E. Banko Viktor Gal G. Papay P. Domsa Jozsef Nemeth Zoltan Vidnyanszky Google Judit Kortvelyes E. Banko Viktor Gal G. Papay P. Domsa Jozsef Nemeth Zoltan Vidnyanszky Google Scholar Judit Kortvelyes E. Banko Viktor Gal G. Papay P. Domsa Jozsef Nemeth Zoltan Vidnyanszky PubMed Judit Kortvelyes E. Banko Viktor Gal G. Papay P. Domsa Jozsef Nemeth Zoltan Vidnyanszky Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Color and luminance increment thresholds in poor readers

The hypotheses of a visual basis to reading disabilities in some children have centered around deficits in the visual processes displaying more transient responses to stimuli although hyperactivity in the visual processes displaying sustained responses to stimuli has also been proposed as a mechanism. In addition, there is clear evidence that colored lenses and/or colored overlays and/or colored backgrounds can influence performance in reading and/or may assist in providing comfortable vision for reading and, as a consequence, the ability to maintain reading for longer. As a consequence, it is surprising that the color vision of poor readers is relatively little studied. We assessed luminance increment thresholds and equi-luminous red-green and blue-yellow increment thresholds using a computer based test in central vision and at 10 degrees nasally employing the paradigm pioneered by King-Smith. We examined 35 poor readers (based on the Neale Analysis of Reading) and compared their performance with 35 normal readers matched for age and IQ. Poor readers produced similar luminance contrast thresholds for both foveal and peripheral presentation compared with normals. Similarly, chromatic contrast discrimination for the red/green stimuli was the same in normal and poor readers. However, poor readers had significantly lower thresholds/higher sensitivity for the blue/yellow stimuli, for both foveal and peripheral presentation, compared with normal readers. This hypersensitivity in blue-yellow discrimination may point to why colored lenses and overlays are often found to be effective in assisting many poor readers.

Hemispheric differences in strong versus weak semantic priming: Evidence from event-related brain potentials

Research with lateralized word presentation has suggested that strong ("close") and weak ("remote") semantic associates are processed differently in the left and right cerebral hemispheres [e.g., Beeman, M. j., & Chiarello, C. (1998). Complementary right- and left-hemisphere language comprehension. Current Directions in Psychological Science, 7(1), 2-8]. Recently, this hypothesis has been challenged [Coney, J. (2002). The effect of associative strength on priming in the cerebral hemispheres. Brain and Cognition, 50(2), 234-241]. We predicted that foveal presentation of strong and weak associates would elicit different patterns of hemispheric activity, as indexed by high-density event-related brain potentials (ERPs), and that source localization of the scalp potentials would help clarify the nature of hemispheric contributions to semantic organization. 128-channel ERPs were recorded in two experiments as subjects performed a lexical decision task. Word trials were equally divided into strongly related, weakly related, and unrelated word pairs. All words were foveally presented. SOA was 800 ms in Experiment 1, and 200 ms in Experiment 2. Topographic analyses revealed medial frontal (MFN) and parietal (N400/LPC) effects for both strong and weak associates. Between approximately 450 and 550 ms, the magnitude of the N400/LPC effect indicated priming for both strong and weak associates over left parietal sites, while priming over right parietal sites was restricted to strongly related word pairs. During this interval, spatiotemporal source modeling showed that these scalp effects were best accounted for by ipsilateral sources in the medial temporal lobe. The observed pattern of asymmetries for strong versus weak associates is not consistent with certain proposals regarding the complementarity of right- and left-hemisphere contributions to semantics. It is, however, consistent with findings from visual half-field studies (Hasbrooke and Chiarello, 1998). We discuss the relevance of these results for theories of hemispheric asymmetry and meta-control in lexical semantic access.

Interhemispheric Integration of Letter Stimuli Presented Foveally or Extra-Foveally

Two experiments are reported in which participants decided whether a single target letter presented below and to the left or right of fixation matched either of two probe letters presented above the fixation cross to the left and right. The level of matching required was either physical (A-A) or abstract (A-a). All three letters were presented either extra-foveally (Experiment 1) or within the fovea (Experiment 2). In both experiments, physical matching was faster than abstract matching. Physical matching was faster within than across visual fields while abstract matching showed the opposite pattern. Matching was faster when the matching probe letter was in the LVF than when it was in the RVF. Importantly, the pattern of results was the same for extra-foveal and foveal presentations, supporting the theory that the representation of the fovea is split down the middle into two visual fields rather then being bilateral. The practical implication following this study is that lateralization studies can be performed with closer to fixation stimuli presentation.

Stimulus–Response Incompatibility Activates Cortex Proximate to Three Eye Fields

We used functional magnetic resonance imaging (fMRI) to investigate cortical activation during the performance of three oculomotor tasks that impose increasing levels of cognitive demand. (1) In a visually guided saccade (VGS) task, subjects made saccades to flashed targets. (2) In a compatible task, subjects made leftward and rightward saccades in response to foveal presentation of the uppercase words "LEFT" or "RIGHT." (3) In a mixed task, subjects made rightward saccades in response to the lowercase word "left" and leftward saccades in response to the lowercase word "right" on incompatible trials (60%). The remaining 40% of trials required compatible responses to uppercase words. The VGS and compatible tasks, when compared to fixation, activated the three cortical eye fields: the supplementary eye field (SEF), the frontal eye field (FEF), and the parietal eye field (PEF). The mixed task, when compared to the compatible task, activated three additional cortical regions proximate to the three eye fields: (1) rostral to the SEF in medial frontal cortex; (2) rostral to the FEF in dorsolateral prefrontal cortex (DLPFC); (3) rostral and lateral to the PEF in posterior parietal cortex. These areas may contribute to the suppression of prepotent responses and in holding novel visuomotor associations in working memory.

Absence of a chromatic linear motion mechanism in human vision

We have investigated motion mechanisms in central and perifoveal vision using two-frame random Gabor kinematograms with isoluminant red-green or luminance stimuli. In keeping with previous results, we find that performance dominated by a linear motion mechanism is obtained using high densities of micropatterns and small temporal intervals between frames, while nonlinear performance is found with low densities and longer temporal intervals [Boulton, J. C., & Baker, C. L. (1994) Proceedings of SPIE, computational vision based on neurobiology, 2054, 124-133]. We compare direction discrimination and detection thresholds in the presence of variable luminance and chromatic noise. Our results show that the linear motion response obtained from chromatic stimuli is selectively masked by luminance noise; the effect is selective for motion since luminance noise masks direction discrimination thresholds but not stimulus detection. Furthermore, we find that chromatic noise has the reverse effect to luminance noise: detection thresholds for the linear chromatic stimulus are masked by chromatic noise but direction discrimination is relatively unaffected. We thus reveal a linear 'chromatic' mechanism that is susceptible to luminance noise but relatively unaffected by color noise. The nonlinear chromatic mechanism behaves differently since both detection and direction discrimination are unaffected by luminance noise but masked by chromatic noise. The double dissociation between the effects of chromatic and luminance noise on linear and nonlinear motion mechanisms is not based on stimulus speed or differences in the temporal presentations of the stimuli. We conclude that: (1) 'chromatic' linear motion is solely based on a luminance signal, probably arising from cone-based temporal phase shifts; (2) the nonlinear chromatic motion mechanism is purely chromatic; and (3) we find the same results for both perifoveal and foveal presentations.