Simulating posterior parietal damage in a biologically plausible framework: Neuropsychological tests of the search over time and space model

Abstract

The search over time and space (sSoTS) model attempts to simulate both the spatial and the temporal aspects of human visual search using spiking level neurons, which incorporate some biologically plausible aspects of neuronal firing. The model contains pools of units that (a) code basic features of objects, presumed to reside in the ventral visual stream, and (b) respond in a feature-independent way to stimulation at their location, presumed to operate in the posterior parietal cortex. We examined the effects of selective lesioning neurons responding to one side of the location map. Unilateral damage introduced spatial biases into selection that affected conjunction more than single-feature search. In addition, there was an impaired ability to segment stimuli over time as well as space (e.g., in preview search). These results match previously reported data on patients with posterior parietal lesions. In addition we show that spatial biases in selection increase under conditions in which there is decreased activity from excitatory neurotransmitters, mimicking effects of reduced arousal. Further simulations explored the effects of time and of visual grouping on extinction, generating predictions that were then tested empirically. The model provides a framework for linking behavioural data from patients with neural-level determinants of visual attention.

Acknowledgments

This work was supported by grants from the Biotechnology and Biological Sciences Research Council (BBSRC), Medical Research Council (MRC), and Stroke Association (UK).

Notes

¹ Simulations with increased numbers of units can take an extremely long time to run.

² This property distinguishes mean field approximations from high-level connectionist simulations, which cannot be derived back to the operation of spiking-level neurons.

³ Posner and Petersen propose that arousal is modulated through the norepinephrine (NE) system. This system is not implemented within sSoTS, and to introduce it would mean changes to the operation of the basic model to accommodate a new neurotransmitter system. The effect of reducing NE is to globally lower activation levels. Our approach was to approximate this by decreasing NMDA activity, which has a similar global effect.

⁴ Note that this was not because the ipsilesional distractor was detected. Performance here was measured according to whether units at the target's position were activated above threshold in the location map.

⁵ It should also be pointed out that negative effects of similarity have not been universally found even in experiments that use localization tasks (e.g., Kitadono & Humphreys, 2007, failed to find the effect across seven experiments in patients showing extinction).