Dorsal Versus Ventral Systems

There is another possible explanation of the gender differences in spatial abilities and forms of creativity. Collaer and Nelson (2002) found that although men were superior in the Judgement of Line Orientation Test, if they altered the response array such that the base angles were not aligned with the bottom edge of the paper, the men's advantage was no longer present. The authors interpreted this finding as suggesting that men used a more global approach and did not solely focus on the lines but also looked at the position of the array on the page. As I mentioned earlier, the visual system is divided into a ventral "what" system and a dorsal "where" system (see Figure 1.1). Object recognition and recognition of familiar faces is more dependent on the ventral "what" system, and functions such as spatial localization are more dependent on the dorsal "where" system. The retina contains special nerve cells (ganglion cells) that become activated when light falls on them, prompting them to send electrical messages along the optic nerve until they reach a relay station in the middle of the brain called the lateral geniculate or optic thalamus (see Figure 6.3). The optic thalamus has two types of neurons: neurons that are large and send information rapidly, called magnocellular, and neurons that are small and send information more slowly, called parvocellular. The visual

Figure 6.3. Diagram of the visual system, demonstrating that stimuli that fall on the retina are projected back by the optic nerves through the optic chiasma to the lateral geniculate or optic thalamus. From the optic thalamus, this information is then projected by way of the geniculo-calcarine tract to the primary visual cortex (V1).

--Thalamus

--Optic Thalamus

<ā€”Vā€” Optic Nerve j (Lateral Geniculate J Nucleus) yr-ā€” Genlculocalcarlne Tract

J-ā€” Calcarine (VI) or Visual Cortex

Retina

Figure 6.3. Diagram of the visual system, demonstrating that stimuli that fall on the retina are projected back by the optic nerves through the optic chiasma to the lateral geniculate or optic thalamus. From the optic thalamus, this information is then projected by way of the geniculo-calcarine tract to the primary visual cortex (V1).

information sent by the parvocellular system to the occipital lobes has high-contrast sensitivity and hence is well suited for detailed (focused) visual analyses, such as that needed to read letters and words. In contrast, the visual information processed by the magnocel-lular system has low-contrast sensitivity that is good for analyzing global configurations. A second series of nerves sends visual information from the optic thalamus to the primary visual cortex in the occipital lobes. The part of the occipital cortex that receives this signal from the optic thalamus is called the calcarine cortex or V1. The cells in this region detect changes in brightness that occurred in specific spatial locations on the retina. The V1, or Brodmann's area 17 (see Figure 1.1), sends this information to the visual association cortex where it is distributed to the ventral "what" and dorsal "where" systems. As I mentioned earlier, men appear to be superior to women in performing spatial "where" analyses, but women are superior to men in facial recognition (Lewin & Herlitz, 2002), which requires "what" analyses. Lesion studies suggest that reading is also performed by the ventral system, and men have two to three times a higher rate of developmental dyslexia than do women (Katusic, Colligan, Barbaresi,

Schaid, & Jacobsen, 2001). It is possible that magnocellular neurons are primarily fed into the dorsal "where" system, and the information that comes into the parvocellular neurons are primarily directed to the ventral "what" system. Perhaps men have superior spatial abilities because they have better developed magnocellular systems and women are superior in face recognition and reading because they have better developed parvocellular systems. Alternatively, the differences between men and women might be related to differences in the development of the ventral versus dorsal systems. In the future, women's and men's brains need to be studied to directly test the hypothesis that men and women have different magnocellular versus parvocellular systems or ventral versus dorsal systems.

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