Specificity of Human Cortical Areas for Reaches and Saccades

Abstract

Electrophysiological studies in monkeys have identified effector-related regions in the posterior parietal cortex (PPC). The lateral intraparietal area, for example, responds preferentially for saccades, whereas the parietal reach region responds preferentially for arm movements. However, the degree of effector selectivity actually observed is limited; each area contains neurons selective for the nonpreferred effector, and many neurons in both areas respond for both effectors. We used functional magnetic resonance imaging to assess the degree of effector preference at the population level, focusing on topographically organized regions in the human PPC [visual area V7, intraparietal sulcus 1 (IPS1), and IPS2]. An event-related design adapted from monkey experiments was used. In each trial, an effector cue preceded the appearance of a spatial target, after which a Go signal instructed subjects to produce the specified movement with the specified effector. Our results show that the degree of effector specificity is limited in many cortical areas and transitions gradually from saccade to reach preference as one moves through the hierarchy of areas in the occipital, parietal, and frontal cortices. Saccade preference was observed in visual cortex, including early areas and V7. IPS1 exhibited balanced activation to saccades and reaches, whereas IPS2 showed a weak but significant preference for reaches. In frontal cortex, areas near the central sulcus showed a clear and absolute preference for reaches, whereas the frontal eye field showed little or no effector selectivity. Although these results contradict many theoretical conclusions about effector specificity, they are compatible with the complex picture arising from electrophysiological studies and also with previous imaging studies that reported mostly overlapping saccade-and arm-related activation. The results are also compatible with theories of efficient coding in cortex.

Functional Heterogeneity in Posterior Parietal Cortex Across Attention and Episodic Memory Retrieval

“…Just as recent studies in visual (Brewer et al 2005;Larsson and Heeger 2006;Sayres and Grill-Spector 2008;Arcaro et al 2009;Kolster et al 2010;Grill-Spector 2010, 2011;Rauschecker et al 2011), frontal (Hagler and Sereno 2006;Kastner et al 2007), and parietal cortex (Levy et al 2007;Swisher et al 2007) illustrate the benefit of multiple experimental techniques for clarifying the complexity of neural organization in both topographic and nontopographic areas, the present study demonstrates the generalizability of this approach to regions involved in multiple aspects of memory and attention. In particular, our results reveal heterogeneity across topographically organized PPC subregions, with 1) IPS0 displaying a distinct pattern of activity from IPS4, IPS5, and SPL1, and 2) IPS1-3 not exhibiting differential responses across retrieval conditions (though future studies with additional power are required to draw definitive conclusions about the relative effects of retrieval outcomes on responses in these areas).…”

Functional Rather than Effector-Specific Organization of Human Posterior Parietal Cortex

“…ROIs were analyzed only in the left hemisphere to account for the fact that participants used their right hand and foot (they obviously moved both eyes during saccades). In previous studies, ROIs V7, IPS1, and IPS2 were mapped using topographic mapping of saccades (i.e., their coordinates were functionally defined by eye movements) (Schluppeck et al, 2005;Silver et al, 2005;Hagler et al, 2007;Levy et al, 2007). V7 is located in Brodmann's area (BA) 19, whereas IPS1 and IPS2 lie anterior to V7 in BA 7.…”

“…Neither region, however, appears to respond exclusively to either effector (Snyder et al, 1997;Andersen and Cui, 2009). Accordingly, research using functional magnetic resonance imaging (fMRI) in humans has revealed a large overlap of PPC activation during the planning of eye and hand movements (Beurze et al, 2007(Beurze et al, , 2009; Levy et al, 2007;Hinkley et al, 2009). However, biases for either eye or arm movements have also been shown in humans (Connolly et al, 2003;Medendorp et al, 2005;Connolly et al, 2007;Hinkley et al, 2009;Van Der Werf et al, 2010), and candidate homologs for macaque areas LIP and PRR have been proposed (Connolly et al, 2003;Schluppeck et al, 2005;Beurze et al, 2007;Hagler et al, 2007).…”

“…However, this conclusion about PPC organization may be premature (Hagler et al, 2007;Levy et al, 2007). Functionally, eye movements are inseparably bound to visual perception and serve a different purpose than movements of any other body part; parietal regions might be differently sensitive to these functions.…”

“…Therefore, specifying a goal-directed limb movement requires additional transformations to compute the spatial relationship between target and effector position, which depends on the current body posture (Buneo et al, 2002;Beurze et al, 2006). Although effector-specific organization of PPC has been questioned on the basis of overlapping activity of eye-and hand-related activity (Hagler et al, 2007;Levy et al, 2007), experiments involving only two effectors do not allow deciding between effector specificity versus other, functionally based explanations as the reason of existing regional activity biases in PPC.…”