Role of Feedforward and Feedback Projections in Figure-Ground Responses

features (e.g. orientation) within a small part of the visual scene. The feature information is further conveyed to surrounding extra-striate areas and from there to the higher level visual areas. In fact, the feedforward projection is dichotomized into two streams. Axons projecting towards areas in the temporal lobe define the ventral pathway (also called as the “what” or “perception” stream) and projections to the parietal areas form the dorsal pathway (also called the “where” or “action” stream). Information flowing to the Visual Cortex – Current Status and Perspectives 390 ventral pathway relates to objects and shapes whereas information conveyed to the dorsal pathway relates to attention and space (see Figure 2). Figure-Ground: Classical psychophysical experiments highlighted that figures emerge from Figure 1. its parts, being perceived as single shapes. Many very well known cases of ambiguity (such as the Rubin vase) exemplify this fact. In these images at any given moment we can perceive as the figure only one of the alternative interpretations, the other becoming the background. The neurophysiological correlates of this phenomenon are discussed here. The neurons in these latter areas have large receptive fields in order to integrate the elementary visual features. A classical receptive field is defined as the region of the visual scene from which a neuronal cell receives direct information by way of feedforward connections. Then, these cells responses to feedforward inputs are more closely related to our daily experience of the external visual world than are the responses in lower order areas since their selectivity is to more elaborated shapes of an object such as a face. That is, what Hubel and Wiesel advanced in 1968 is essentially true: receptive fields of cells at one level of the visual system are formed from inputs by cells at a lower level. In this way, small, simple receptive fields combine to form large, complex receptive fields. Feedforward projections are therefore the anatomical substrate for the initial transient response of a neuron to a stimulus, and determine the size and tuning properties of the stimulus evoked response. For instance, the orientation tuning of V1 neurons is predominantly determined by feedforward inputs (Miller, 2003) and by the biophysical membrane properties of the cells (Cardin et al., 2007). The spatial arrangement of the receptive fields of cells in the primary visual cortex follows a retino-topical organization and provides a topographic map of the visual world. Simple cells have an elongated receptive field structure, with an excitatory central oval and an inhibitory surrounding region (Hubel & Wiesel, 1968). In order to excite these cells stimuli need to have a particular orientation or direction. In the case of V1 complex cells, the receptive fields have no clear separation of excitatory and inhibitory regions. To excite these cells an oriented stimulus may need to move in a particular direction and might also need to be of a particular length. Beside excitatory neurons, inhibitory cells are also tuned to orientation and spatial frequency (Cardin et al., 2007). Thus, V1 cells respond selectively to simple, rather abstract features that make up an object within a small part of the visual scene mainly by reason of their connections with striate projecting neurons. Role of Feedforward and Feedback Projections in Figure-Ground Responses 391 Schematic representation of the main information pathways of the visual system and its Figure 2. anatomical location. Sensory information is conveyed from the retina to the primary visual cortex (V1) from where it is distributed across the higher visual areas via the extra-striate areas. The dorsal stream begins with V1, goes through visual area V2, then to the dorsomedial area and visual area MT (also known as V5) and to the posterior parietal cortex. The ventral stream begins with V1, goes through visual area V2, then through visual area V4, and to the inferior temporal cortex. Top-down information from high-level visual areas towards low-level areas is mediated by feedback connections. V1 MT