Perception and action have been traditionally considered separate domains, each of them being implemented in separate anatomical and functional brain sectors. These sectors are serially organized: individuals first perceive, then act. In primates, the paradigmatic sensory modality for the study of the mechanisms underlying perception is vision. Visual information is processed both serially and along parallel pathways. Serial organization fits well the classical concept that information travels from the periphery to progressively more complex "association" areas where perception occurs and then proceeds to output channels for action. In contrast, parallel organization of visual processing needs explanation.

A particularly influential account of why there is parallel processing of visual information in the primate visual cortex is that proposed by Ungerleider and Mishkin (1982). According to these authors, the visual cortical areas are organized in two separate streams of visual information. A dorsal stream, which includes visual areas MT, MST, FST, V3A, and V6, and culminates in the inferior parietal lobule, and a ventral stream, which includes visual areas V3 and V4, and culminates in the inferior temporal cortex. The dorsal stream is responsible for perception of space, while the ventral stream for object perception.

An equally influential and radically different view was advanced by Milner and Goodale (Goodale and Milner, 1992; Milner and Goodale, 1995). In accord with Mishkin and Ungerleider (1982) they maintain that there is a fundamental functional difference between the dorsal and ventral stream. They deny, however, that the difference is in the resulting percept (space vs. object). According to Goodale and Milner the difference is in the output characteristics of the two cortical visual streams. The ventral stream is fundamental for perception. The dorsal stream, in contrast, processes visual stimuli to provide high order visual information for the control of action, but it is not involved in perception. A similar view was independently proposed by Jeannerod (1994, 1997). According to Jeannerod, the ventral stream is responsible for the "semantic mode" of object representation, while the dorsal stream is responsible for the "pragmatic mode" of stimulus processing. The semantic mode of object representation refers to object analysis described in object-centered coordinates. The pragmatic mode indicates the type of processing that stimuli have to undergo for action organization. Although the distinction between a semantic and a pragmatic system proposed by Jeannerod appears to be more cautious than that of Milner and Goodale, the essence of the two proposals is very similar.

The aim of the present chapter is two-folded. First it aims to show that perception requires action. This is most evident for some types of visual percept (e.g. space perception and action perception). Second, it aims to show that the distinction of the cortical visual processing into two streams is insufficient and leads to possible misunderstandings on the true nature of perceptual processes.

I will briefly review empirical findings suggesting that visual processing is carried out along three distinct streams (see Figure 17-1). Two of them include the parietal lobe, one includes the inferotemporal lobe. These three streams are qualified as dorso-dorsal, ventro-dorsal and ventral streams (for a detailed analysis of the dorsal streams, see Rizzolatti and Matelli, 2003; Rozzi et al., 2006).

On-lme control of action

Dorso-dorsal Stream

Ventro-dorsal Strea m

Space perception Representation of action Action recognition

Ventral Stream

Object perception

Figure 17-1. The three Visual Streams. Lateral and mesial view of the macaque monkey brain showing the main connectivity along the dorso-dorsal, ventro-dorsal and ventral streams of visual processing and their hypothesized functions. Abbreviations: C=central sulcus; Ca=calcarine fissure; CC=corpus callosus; cg=cingulated gyrus; dPM=dorsal premotor cortex; IA=inferior arcuate sulcus; IO=inferior orbital sulcus; IPL=inferior parietal lobule; IT=inferior temporal cortex; L=lateral fissure; LPC=lateral prefrontal cortex; Lu=lunate sulcus; MI=primary motor cortex; P=principal sulcus; pre-SMA=pre-supplementary motor area; SPL=superior parietal lobule; ST=superior temporal sulcus; VPM=ventral premotor cortex.

The dorso-dorsal stream has the characteristics suggested by Milner and Goodale and Jeannerod when they describe the dorsal stream as a whole. It appears to be possibly the only stream that is not directly related to perception. The ventro-dorsal stream, which will be the main focus of this chapter, is responsible for the organization of actions directed towards objects, but also for space perception and action perception. Finally, the ventral stream is responsible for the organization of actions following object categorization, and for object semantics. In the ventral stream are located the semantic trees linking the analyzed objects to individual's semantic knowledge. All three visual streams terminate into frontal cortical areas endowed with different degrees of complexity.

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