THE POSTERIOR PARIETAL CORTEX AND VISION

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The posterior parietal cortex has been well studied in visual neuroscience mainly because it is believed that it plays a central role in linking the visual system to ...
THE POSTERIOR PARIETAL CORTEX AND VISION, Edward J Tehovnik, Brain Institute, Natal, RN, 59056-450, Brazil, tehovnikej@ gmail.com, April 2014. The posterior parietal cortex has been well studied in visual neuroscience mainly because it is believed that it plays a central role in linking the visual system to the oculomotor system (Schiller and Tehovnik 2001). Vernon Mountcastle performed pioneering studies in the parietal cortex that lead him to propose that the cerebral cortex is organized according to columns (Mountcastle 1978). He was one of the first to show that the posterior parietal cortex, which is composed of area 7a, the lateral intraparietal area, and the ventral intraparietal area, contains neurons that are modulated by changes in the orbital position of the eyes as monkeys actively fixated a visual target for a liquid reward (Andersen and Mountcastle 1983; Lynch et al. 1977; Mountcastle et al. 1981). Moreover, he showed that this area has cells that are modulated during visually-guided reaching (Mountcastle et al. 1975). Mountcastle hypothesized that the posterior parietal cortex is not only involved in perception but that it contributes directly to the generation of movement. This notion later evolved into the now famous controversy between Richard Andersen and Mickey Goldberg. Richard Andersen proposed that the posterior parietal cortex generates the ‘intention’ to move, whereas Mickey Goldberg asserted that it mediates ‘attention’ emphasizing its role in visual perception. After over 30 years this dispute is still largely unresolved. As mentioned, the neurons of the posterior parietal cortex are modulated by the active fixation of visual targets. In addition these cells can have visual receptive fields whose sensitivity to visual stimuli too varies with changes in eye position (Andersen et al. 1985; 1990; Andersen and Mountcastle 1983). This finding has been used to suggest that the posterior parietal cortex combines a sensory signal with an eye position signal to compute a visual target position in space (Zipser and Andersen 1988) something that would be necessary when one reaches for a cup of coffee, for instance. By varying the position of either the head or the forelimbs during visually-guided reaching it was established that the posterior parietal cortex mediates vision according to an oculocentric scheme (Batista et al. 1999; Brochie et al. 1995).

Electrical stimulation of the posterior parietal cortex has revealed that both fixed vector and convergent saccadic eye movements can be evoked from this area (Shibutani et al. 1984; Kurylo and Skevenski 1991; Thier and Andersen 1998). By delivering electrical stimulation to this region in head-free monkeys, however, it was found that the elicited saccadic eye movements were mainly oculocentric (Constantin et al. 2007). Furthermore, when cells modulated by visual fixation were stimulated electrically, a monkey’s fixation of a visual target was prolonged, and such stimulation during the execution of a paired-target selection task caused an animal to avoid the visual target positioned in the field coded by the stimulated cells (Schiller and Tehovnik 2001). So what happens when the posterior parietal cortex in inactivated? Schiller and Tehovnik (2003) injected 1 µl of 0.5 µg/µl concentration of muscimol (a GABA agonist) into this region in monkeys. Although such injections made into V1 devastated a monkey’s ability to perform a visual discrimination task the effects observed following posterior parietal injection were negligible. Even on a paired-target selection task, there was no shift in target choice away from the target in the hemifield represented by the injection site when using up to 1 µl of muscimol. This result differs somewhat for that of Wardak and colleagues (2002) who reported a deficit in target selection. We suspect that the effects observed by Wardak et al. (2002) were due to tissue damage (Fig. 1B of Wardak et al. 2002) and the spread of the chemical agent. On the second point it was estimated that a 0.5 to 1.0 µl injection (@ 0.5 µg/µl) of muscimol disables a volume of cortex of 1.5-2.5 mm from the injection tip (Schiller and Tehovnik 2003). In the experiments of Wardak et al (2002) the amount of muscimol administered was over 20 times greater (i.e. 18 µg delivered in an 18 µl solution versus 0.5 µg delivered in a 1 µl solution); thence, it is very possible that the drug was spreading well beyond the parietal cortex. To further bolster the view that GABA drugs do not interfere with visual processing in the posterior parietal cortex, injections of bicuculline (a GABA antagonist) had absolutely no effect on visual discrimination and target choice (Schiller and Tehovnik 2003). As mentioned, the precise function of the parietal cortex in vision is still unclear. For example, it was shown recently that the fixation modulation of cells in the posterior parietal cortex lags the offset of a saccadic eye

2 movement to a new fixation location by some 200 ms (Morris et al. 2012; Xu et al. 2012; also see Andersen and Mountcastle 1983: Figs. 1- 4, 10). This delay has been deemed too great for proprioception to be used in the assessment of the location of visual targets using a spatial coordinate scheme (Xu et al. 2012). Goldberg and colleagues have therefore suggested that target location is instead determined by an ‘efferents copy’ signal shared by the posterior parietal cortex (Duhamel et al. 1992; Kusunoki and Goldberg 2003; Xu et al. 2012). Nevertheless, this idea has its problems as well given that it is unclear whether the signal codes for target position or the endpoint of saccadic eye movements directed toward visual targets (Tolias et al. 2001; Zirnsak et al. 2014). It has been found that electrical stimulation in the posterior parietal cortex can both inhibit and facilitate target choice depending on the site of stimulation (Schiller and Tehovnik 2001). Just how these results are related to the ‘efferents copy’ signal described by Goldberg and colleagues awaits clarification. Conclusions 1. The posterior parietal cortex mediates visual fixation as demonstrated by single cell recording and electrical stimulation. Also an oculocentric code is operative in this part of the brain. 2. Lesions of the posterior parietal cortex using chemical agents produce few deficits in visual behavior including target selection. 3. The posterior parietal cortex is believed to have access to an ‘efferents copy’ signal for computing the future position of visual targets. Some believe, however, that this signal codes the endpoint of saccadic eye movements made to visual targets and therefore its role in predicting future target position is limited. 4. Electrical stimulation of the posterior parietal cortex can both inhibit and facilitate target selection. Deducing the mechanism by which this occurs needs further study.

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