Thomas Habekost

A Neural Theory of Visual Attention: Bridging Cognition and Neurophysiology

A neural theory of visual attention (NTVA) is presented. NTVA is a neural interpretation of C. Bundesens (1990) theory of visual attention (TVA). In NTVA, visual processing capacity is distributed across stimuli by dynamic remapping of receptive fields of cortical cells such that more processing resources (cells) are devoted to behaviorally important objects than to less important ones. By use of the same basic equations used in TVA, NTVA accounts for a wide range of known attentional effects in human performance (reaction times and error rates) and a wide range of effects observed in firing rates of single cells in the primate visual system. NTVA provides a mathematical framework to unify the 2 fields of research--formulas bridging cognition and neurophysiology.

Principles of Visual Attention: Linking Mind and Brain

1. Introduction PART I - THE PSYCHOLOGY OF VISUAL ATTENTION 2. Psychological research on visual attention 3. A psychological theory of visual attention (TVA) 4. Explaining divided attention by TVA 5. Explaining focused attention by TVA PART II - THE NEUROPHYSIOLOGY OF VISUAL ATTENTION 6. Effects of visual attention in single neurons 7. A neural theory of visual attention (NTVA) 8. Explaining attentional effects in single neurons by NTVA PART III - THE ANATOMY OF VISUAL ATTENTION 9. Brain imaging of visual attention 10. Disturbances of visual attention 11. TVA-based assessment CONCLUSION 12. A unified theory of visual attention

Too Little, Too Late: Reduced Visual Span and Speed Characterize Pure Alexia

Whether normal word reading includes a stage of visual processing selectively dedicated to word or letter recognition is highly debated. Characterizing pure alexia, a seemingly selective disorder of reading, has been central to this debate. Two main theories claim either that 1) Pure alexia is caused by damage to a reading specific brain region in the left fusiform gyrus or 2) Pure alexia results from a general visual impairment that may particularly affect simultaneous processing of multiple items. We tested these competing theories in 4 patients with pure alexia using sensitive psychophysical measures and mathematical modeling. Recognition of single letters and digits in the central visual field was impaired in all patients. Visual apprehension span was also reduced for both letters and digits in all patients. The only cortical region lesioned across all 4 patients was the left fusiform gyrus, indicating that this region subserves a function broader than letter or word identification. We suggest that a seemingly pure disorder of reading can arise due to a general reduction of visual speed and span, and explain why this has a disproportionate impact on word reading while recognition of other visual stimuli are less obviously affected.

Sustained attention, attentional selectivity, and attentional capacity across the lifespan

Changes in sustained attention, attentional selectivity, and attentional capacity were examined in a sample of 113 participants between the ages of 12 and 75. To measure sustained attention, we employed the sustained-attention-to-response task (Robertson, Manly, Andrade, Baddeley, & Yiend, Neuropsychologia 35:747–58, 1997), a short continuous-performance test designed to capture fluctuations in sustained attention. To measure attentional selectivity and capacity, we employed a paradigm based on the theory of visual attention (Bundesen, Psychological Review 97:523–547, 1990), which enabled the estimation of parameters related to attentional selection, perceptual threshold, visual short-term memory capacity, and processing capacity. We found evidence of age-related decline in each of the measured variables, but the declines varied markedly in terms of magnitude and lifespan trajectory. Variables relating to attentional capacity showed declines of very large effect sizes, while variables relating to attentional selectivity and sustained attention showed declines of medium to large effect sizes, suggesting that attentional control is relatively preserved in older adults. The variables relating to sustained attention followed a U-shaped, curvilinear trend, and the variables relating to attentional selectivity and capacity showed linear decline from early adulthood, providing further support for the differentiation of attentional functions.

Patient assessment based on a theory of visual attention (TVA): subtle deficits after a right frontal-subcortical lesion

We report on a patient who complained of reduced awareness in the left visual field, but showed no visual neglect or extinction in clinical testing. By MR scanning, the brain damage was localized to the right basal ganglia, also involving structures in right frontal cortex. Using psychophysical testing and mathematical modeling based on Bundesens theory of visual attention [TVA; Psychol. Rev. 97 (1990) 523], the patients subjective experience of attentional disturbance was confirmed, and the deficit was specified into several components. At very short exposure durations, two effects were shown. The detection threshold was elevated, particularly in the left visual field, and stimuli in this side were given less attentional weight. In addition, the capacity of visual short-term memory (VSTM) was markedly reduced in both visual fields. The robustness of the test results was evaluated by bootstrap analysis. The study demonstrates the sensitivity and specificity gained by combining psychophysical testing with TVA modeling in the analysis of visual attention disorders. Extending the results of a pioneer study of parietal neglect patients by Duncan et al. [J. Exp. Psychol.: Gen. 128 (1999) 450], this study demonstrates the strength of the method in a single case, with a lesion outside parietal cortex, and only minor clinical symptoms.

A neural theory of visual attention and short-term memory (NTVA)

The neural theory of visual attention and short-term memory (NTVA) proposed by Bundesen, Habekost, and Kyllingsbæk (2005) is reviewed. In NTVA, filtering (selection of objects) changes the number of cortical neurons in which an object is represented so that this number increases with the behavioural importance of the object. Another mechanism of selection, pigeonholing (selection of features), scales the level of activation in neurons coding for a particular feature. By these mechanisms, behaviourally important objects and features are likely to win the competition to become encoded into visual short-term memory (VSTM). The VSTM system is conceived as a feedback mechanism that sustains activity in the neurons that have won the attentional competition. NTVA accounts both for a wide range of attentional effects in human performance (reaction times and error rates) and a wide range of effects observed in firing rates of single cells in the primate visual system.

Finding Wally: Prism adaptation improves visual search in chronic neglect

Several studies have found that visuo-motor adaptation to rightward deviating prismatic goggles (prism adaptation) can alleviate symptoms of neglect after brain damage, but the long-term effect and clinical relevance of this rehabilitation approach have been questioned. In particular, the effect on visual search performance is controversial. In the present study 6 patients with chronic spatial neglect due to rightsided focal brain damage were given 20 sessions of prism adaptation over a period of two weeks. These patients, as well as a matched control group of neglect patients (n=5), were tested using a variety of effect measures with special emphasis on visual search at baseline, shortly after training, and five weeks later. A positive and very consistent long-term effect of prism adaptation was found across clinical tests of neglect, lateral bias of eye movements, and measures of everyday function, including subjective reports. The results show that prism adaptation can provide durable and clinically significant alleviation of neglect symptoms, even in the stable phase of recovery.

Visual processing in pure alexia: A case study

Whether pure alexia is a selective disorder that affects reading only, or if it reflects a more general visual disturbance, is highly debated. We have investigated the selectivity of visual deficits in a pure alexic patient (NN) using a combination of psychophysical measures, mathematical modelling and more standard experimental paradigms. NNs naming and categorization of line drawings were normal with regards to both errors and reaction times (RTs). Psychophysical experiments revealed that NNs recognition of single letters at fixation was clearly impaired, and recognition of single digits was also affected. His visual apprehension span was markedly reduced for letters and digits. His reduced visual processing capacity was also evident when reporting letters from words. In an object decision task with fragmented pictures, NNs performance was abnormal. Thus, even in a pure alexic patient with intact recognition of line drawings, we find evidence of a general visual deficit not selective to letters or words. This finding is important because it raises the possibility that other pure alexics might have similar non-selective impairments when tested thoroughly. We argue that the general visual deficit in NN can be accounted for in terms of inefficient build-up of sensory representations, and that this low level deficit can explain the pattern of spared and impaired abilities in this patient.

Visual attention capacity: a review of TVA-based patient studies.

Psychophysical studies have identified two distinct limitations of visual attention capacity: processing speed and apprehension span. Using a simple test, these cognitive factors can be analyzed by Bundesens Theory of Visual Attention (TVA). The method has strong specificity and sensitivity, and measurements are highly reliable. As the method is theoretically founded, it also has high validity. TVA-based assessment has recently been used to investigate a broad range of neuropsychological and neurological conditions. We present the method, including the experimental paradigm and practical guidelines to patient testing, and review existing TVA-based patient studies organized by lesion anatomy. Lesions in three anatomical regions affect visual capacity: The parietal lobes, frontal cortex and basal ganglia, and extrastriate cortex. Visual capacity thus depends on large, bilaterally distributed anatomical networks that include several regions outside the visual system. The two visual capacity parameters are functionally separable, but seem to rely on largely overlapping brain areas.

Visual attention capacity after right hemisphere lesions

Recently there has been a growing interest in visual short-term memory (VSTM) including the neural basis of the function. Processing speed, another main aspect of visual attention capacity, has received less investigation. For both cognitive functions human lesion studies are sparse. We used a whole report experiment for estimation of these two parameters in 22 patients with right side stroke. Psychophysical performance was analyzed using Bundesens [Bundesen, C. (1990). A theory of visual attention. Psychological Review, 97, 523-547] Theory of Visual Attention (TVA) and compared statistically to lesion location and size measured by MRI. Visual processing speed was impaired in the contralesional hemifield for most patients, but typically preserved ipsilesionally, even after large cortico-subcortical lesions. When bilateral deficits in processing speed occurred, they were related to damage in the right middle frontal gyrus or leukoaraiosis. The storage capacity of VSTM was also normal for most patients, but deficits were found after severe leukoaraiosis or large strokes extending deep into white matter. Thus, the study demonstrated the importance of white-matter connectivity for both VSTM capacity and ipsilesional processing speed. The study also showed that lesions in a large region of the right hemisphere, including the putamen, insula, and inferior frontal cortex, do not lead to general deficits in the capacity of visual attention.