Jennifer K. E. Steeves

Tms to the lateral occipital cortex disrupts object processing but facilitates scene processing

The study of brain-damaged patients and advancements in neuroimaging have lead to the discovery of discrete brain regions that process visual image categories, such as objects and scenes. However, how these visual image categories interact remains unclear. For example, is scene perception simply an extension of object perception, or can global scene “gist” be processed independently of its component objects? Specifically, when recognizing a scene such as an “office,” does one need to first recognize its individual objects, such as the desk, chair, lamp, pens, and paper to build up the representation of an “office” scene? Here, we show that temporary interruption of object processing through repetitive TMS to the left lateral occipital cortex (LO), an area known to selectively process objects, impairs object categorization but surprisingly facilitates scene categorization. This result was replicated in a second experiment, which assessed the temporal dynamics of this disruption and facilitation. We further showed that repetitive TMS to left LO significantly disrupted object processing but facilitated scene processing when stimulation was administered during the first 180 msec of the task. This demonstrates that the visual system retains the ability to process scenes during disruption to object processing. Moreover, the facilitation of scene processing indicates disinhibition of areas involved in global scene processing, likely caused by disrupting inhibitory contributions from the LO. These findings indicate separate but interactive pathways for object and scene processing and further reveal a network of inhibitory connections between these visual brain regions.

Contrast letter thresholds in the non-affected eye of strabismic and unilateral eye enucleated subjects

To investigate the effects of visual disruption on contrast letter thresholds of the non-affected eye, subjects with one eye enucleated, strabismic subjects using the non-deviating eye and normal control subjects were asked to identify letters on eye charts and single letter cards which varied in contrast (between 4 and 96%) and size. At all contrast, contrast letter acuity of eye enucleated subjects was superior to both normal control subjects and strabismic subjects. Early onset strabismic subjects (onset < 24 months) showed inferior performance to normal control subjects at all contrasts of 25% and above. Late onset strabismic subjects showed normal performance at all contrasts, except for high contrast single letters, where performance was inferior to normal control subjects. Further, for all subjects groups, performance on letter charts was similar to performance on single letter cards. We conclude that disruption to the visual system caused by eye enucleation or strabismus is not equivalent. These differences may be due to intrinsic differences between the visual systems of eye enucleated subjects and strabismic subjects and/or to the profound differences in deprivation caused by the two conditions.

Transcranial magnetic stimulation to the transverse occipital sulcus affects scene but not object processing

Traditionally, it has been theorized that the human visual system identifies and classifies scenes in an object-centered approach, such that scene recognition can only occur once key objects within a scene are identified. Recent research points toward an alternative approach, suggesting that the global image features of a scene are sufficient for the recognition and categorization of a scene. We have previously shown that disrupting object processing with repetitive TMS to object-selective cortex enhances scene processing possibly through a release of inhibitory mechanisms between object and scene pathways [Mullin, C. R., & Steeves, J. K. E. TMS to the lateral occipital cortex disrupts object processing but facilitates scene processing. Journal of Cognitive Neuroscience, 23, 4174–4184, 2011]. Here we show the effects of TMS to the transverse occipital sulcus (TOS), an area implicated in scene perception, on scene and object processing. TMS was delivered to the TOS or the vertex (control site) while participants performed an object and scene natural/nonnatural categorization task. Transiently interrupting the TOS resulted in significantly lower accuracies for scene categorization compared with control conditions. This demonstrates a causal role of the TOS in scene processing and indicates its importance, in addition to the parahippocampal place area and retrosplenial cortex, in the scene processing network. Unlike TMS to object-selective cortex, which facilitates scene categorization, disrupting scene processing through stimulation of the TOS did not affect object categorization. Further analysis revealed a higher proportion of errors for nonnatural scenes that led us to speculate that the TOS may be involved in processing the higher spatial frequency content of a scene. This supports a nonhierarchical model of scene recognition.

Early unilateral enucleation disrupts motion processing.

Previous research has shown that unilaterally enucleated observers demonstrate better luminance-defined form perception compared to monocularly viewing controls, and similar performance to control observers viewing binocularly (Vision Res. 37(17) (1997) 2465). In Experiment 1 we asked whether the perception of form, where form is defined by other attributes than luminance, is also improved compared to monocularly viewing controls. We tested 16 enucleated observers and 25 controls viewing monocularly and binocularly for their ability to detect and recognize form from texture (texture-defined (TD) form) and form from motion (motion-defined (MD) form). There was no difference between the three groups for TD form perception. However, enucleated observers had significantly poorer MD form perception than did binocularly viewing controls. In Experiment 2 we asked whether poor performance on the perception of MD form might be due to a general reduction in motion processing abilities. To examine this possibility, we used a motion coherence task. We tested eight unilaterally enucleated and 14 monocularly and binocularly viewing control observers on a horizontal coherent motion discrimination task. The monocularly viewing controls showed no naso-temporal asymmetry in direction discrimination for coherent motion. In contrast, the enucleated group showed an asymmetry in direction discrimination where temporalward motion coherence thresholds were significantly higher than those for nasalward motion. These latter findings are discussed in terms of the absence of binocular competition during the development of motion processing pathways.

A comparison of contrast letter thresholds in unilateral eye enucleated subjects and binocular and monocular control subjects

We previously reported that unilaterally eye enucleated subjects show superior contrast letter acuity to normally sighted monocular viewing control subjects. We suggested that reorganization of the visual system in the enucleated subjects may compensate for their loss of binocularity. Here we measured contrast letter acuity in normally sighted binocular control subjects and compared these results to previously published results of eye enucleated subjects and monocular viewing control subjects. We found equivalent performance between enucleated subjects and binocular control subjects, suggesting that performance of enucleated subjects might be due to some form of neural summation.

Consecutive TMS-fMRI Reveals an Inverse Relationship in BOLD Signal between Object and Scene Processing

The human visual system is capable of recognizing an infinite number of scenes containing an abundance of rich visual information. There are several cortical regions associated with the representation of a scene, including those specialized for object processing (the lateral occipital area [LO]) and for the spatial layout of scenes (the parahippocampal place area). Although behavioral studies have demonstrated that these image categories (scenes and objects) exert an influence on each other such that scene context can facilitate object identification or that scene categorization can be impaired by the presence of a salient object, little is known about the apparent cortical interactions involved in building the conscious representation of a complete scene. It has been shown that transcranial magnetic stimulation (TMS) to the left LO disrupts object categorization but facilitates scene categorization. Here, we show that this effect is also reflected by changes in the BOLD signal such that TMS to the left LO decreases BOLD signal at the stimulation site (LO) while viewing objects and increases BOLD signal in the left PPA when viewing scenes. This suggests that these regions, although likely not on a strict hierarchy of bottom-up coding, share functional communication likely in the form of inhibitory connections.

Accuracy of estimating time to collision using only monocular information in unilaterally enucleated observers and monocularly viewing normal controls

Since individuals who have lost an eye in early life rely on monocular information, one asked if they would better estimate the time to collision (TTC) with an approaching object based on the monocular cue [(θ/(dθ/dt), i.e. tau] than a control group using only monocular information. Estimates of TTC were measured with a simulated approaching textured object using a staircase procedure. Seven adult observers who were unilaterally enucleated at an early age were compared with 18 normally sighted control observers who viewed the stimuli monocularly. Consistent with previous findings, the majority of the controls (13/18) underestimated TTC. Three enucleated observers had larger estimation errors than the 95% confidence interval of the mean of the control group. One enucleated observer was unable to give reliable results. These results suggest that unilaterally enucleated observers cannot estimate TTC more accurately (and may even be worse) than normal controls when estimates are based on monocular information alone. Further, the majority (83%) of enucleated observers were influenced by perceived distance information derived from the objects initial size when estimating TTC with an approaching object. The use of this other optical variable could account for their reduction in performance. It was suggested that in every day life enucleated individuals make use of as many optical variables as possible to partially compensate for the lack of binocularity.

Monocular horizontal OKN in observers with early- and late-onset strabismus

Several reports on monocular optokinetic nystagmus (OKN) in observers with strabismus have found that asymmetry of OKN tends to occur in both eyes of observers with an early onset of strabismus but only in the deviating eye of those with a later onset of strabismus. Our objective was to quantify and compare the magnitude of the OKN asymmetry in each eye as a function of observers age at onset of strabismus. We studied monocular OKN in ten observers with early-onset (up to 24 months of age), seven observers with late-onset (after 24 months of age) unilateral strabismus, and 12 normally sighted control observers. In the deviating eye, observers with early-onset strabismus showed large OKN asymmetries in favour of nasalward motion while observers with late-onset strabismus showed smaller OKN asymmetries in that eye. The majority of early- and late-onset observers showed near normal OKN in the non-deviating eye although the early-onset observers showed bilateral asymmetries more often. These findings may be due to both age at onset of strabismus and chronological age and are discussed in terms of the issue of plasticity or recovery of function.

Plasticity in Sensory Systems

Plasticity is a fundamental property of neural development and learning in living organisms. It also contributes to problems associated with aging and degenerative processes. Understanding neural plasticity has huge implications for those seeking to recover from brain injury or sensory deprivation and regular folk vying to improve their skills and ability. Centered on three themes, this book explores the latest research in plasticity in sensory systems, with a focus primarily on visual and auditory systems. It covers a breadth of recent scientific study within the field including research on healthy systems and diseased models of sensory processing. Topics include visual and visuomotor learning, models of how the brain codes visual information, sensory adaptations in vision and hearing as a result of partial or complete visual loss in childhood, plasticity in the adult visual system, and plasticity across the senses, as well as new techniques in vision recovery, rehabilitation, and sensory substitution of other senses when one sense is lost. This unique edited volume, the fruit of an International Conference on Plastic Vision held at York University, Toronto, will provide students and scientists with an overview of the ongoing research related to sensory plasticity and perspectives on the direction of future work in the field.

Optimal Audiovisual Integration in People with One Eye

People with one eye show altered sensory processing. Such changes might reflect a central reweighting of sensory information that might impact on how multisensory cues are integrated. We assessed whether people who lost an eye early in life differ from controls with respect to audiovisual integration. In order to quantify the relative weightings assigned to each sensory system, participants were asked to spatially localize audiovisual events that have been previously shown to be optimally combined and perceptually fused from the point of view of location in a normal population, where the auditory and visual components were spatially disparate. There was no difference in the variability of localizing unimodal visual and auditory targets by people with one eye compared to controls. People with one eye did however, demonstrate slower reaction times to localize visual stimuli compared to auditory stimuli and were slower than binocular and eye-patched control groups. When localizing bimodal targets, the weightings assigned to each sensory modality in both people with one eye and controls were predictable from their unimodal performance, in accordance with Maximum Likelihood Estimation and the time it took all three groups to localize the bimodal targets was faster than for vision alone. Regardless of demonstrating a longer response time to visual stimuli, people with one eye appear to integrate the auditory and visual components of multisensory events optimally when determining spatial location.