Biased associative representations in parietal cortex.

Neurons in cortical sensory areas respond selectively to sensory stimuli, and the preferred stimulus typically varies among neurons so as to continuously span the sensory space. However, some neurons reflect sensory features that are learned or task dependent. For example, neurons in the lateral intraparietal area (LIP) reflect learned associations between visual stimuli. One might expect that roughly even numbers of LIP neurons would prefer each set of associated stimuli. However, in two associative learning experiments and a perceptual decision experiment, we found striking asymmetries: nearly all neurons recorded from an animal had a similar order of preference among associated stimuli. Behavioral factors could not account for these neuronal biases. A recent computational study proposed that population-firing patterns in parietal cortex have one-dimensional dynamics on long timescales, a possible consequence of recurrent connections that could drive persistent activity. One-dimensional dynamics would predict the biases in selectivity that we observed.

Direction selectivity of neurons in the macaque lateral intraparietal area.

The lateral intraparietal area (LIP) of the macaque is believed to play a role in the allocation of attention and the plan to make saccadic eye movements. Many studies have shown that LIP neurons generally encode the static spatial location demarked by the receptive field (RF). LIP neurons might also provide information about the features of visual stimuli within the RF. For example, LIP receives input from cortical areas in the dorsal visual pathway that contain many direction-selective neurons. Here we examine direction selectivity of LIP neurons. Animals were only required to fixate while motion stimuli appeared in the RF. To avoid spatial confounds, the motion stimuli were patches of randomly arrayed dots that moved with 100% coherence in eight different directions. We found that the majority (61%) of LIP neurons were direction selective. The direction tuning was fairly broad, with a median direction-tuning bandwidth of 136 degrees. The average strength of direction selectivity was weaker in LIP than that of other areas of the dorsal visual stream but that difference may be because of the fact that LIP neurons showed a tonic offset in firing whenever a visual stimulus was in the RF, independent of direction. Direction-selective neurons do not seem to constitute a functionally distinct subdivision within LIP, because those neurons had robust, sustained delay-period activity during a memory delayed saccade task. The direction selectivity could also not be explained by asymmetries in the spatial RF, in the hypothetical case that the animals attended to slightly different locations depending on the direction of motion in the RF. Our results show that direction selectivity is a distinct attribute of LIP neurons in addition to spatial encoding.

Serial attention mechanisms in visual search: a direct behavioral demonstration.

In visual search, inefficient performance of human observers is typically characterized by a steady increase in reaction time with the number of array elements-the so-called set-size effect. In general, set-size effects are taken to indicate that processing of the array elements depends on limited-capacity resources, that is, it involves attention. Contrasting theories have been proposed to account for this attentional involvement, however. While some theories have attributed set-size effects to the intervention of serial attention mechanisms, others have explained set-size effects in terms of parallel, competitive architectures. Conclusive evidence in favor of one or the other notion is still lacking. Especially in view of the wide use of visual search paradigms to explore the functional neuroanatomy of attentional mechanisms in the primate brain, it becomes essential that the nature of the attentional involvement in these paradigms be clearly defined at the behavioral level. Here we report a series of experiments showing that highly inefficient search indeed recruits serial attention deployment to the individual array elements. In addition, we describe a number of behavioral signatures of serial attention in visual search that can be used in future investigations to attest a similar involvement of serial attention in other search paradigms. We claim that only after having recognized these signatures can one be confident that truly serial mechanisms are engaged in a given visual search task, thus making it amenable for exploring the functional neuroanatomy underlying its performance.