Physiology

Electro-physiologists are m ost concerned with the biological architecture o f the visual system, asking: w hat biologically plausible structure could be responsible for coding optic flow ? O ne influential proposal has global m otion detection accom plished in m ultiple stages o f processing (Van Essen & M aunsell, 1983), Early m otion-sensitive cells in V I have spatially restricted receptive fields, and

so are ill suited to respond unam biguously to the m ovem ent o f objects that exceed their field o f view (the aperture problem , described above).

A natom ical investigation o f the prim ate visual architecture, prim arily in the m acaque anim al m odel, has revealed parallel pathw ays leading from the retinae to the cortex via the lateral geniculate nucleus (LG N ). T hese pathw ays are referred to as the M and P pathw ays, and account for approxim ately 90% o f the projections from the retinae (Shapley & Perry, 1986). Sub-cortical ganglion cells, m agnocellular and parvocellular neurons, w hile generally responsive to tem poral m odulation when a stim ulus is m oved through their receptive field, do not show strong directional selectivity. C ortical area V I is the site at which this property, so essential for higher order m otion processing, is first encountered (Hubei & W iesel, 1968; Colby et al., 1993; D e V alois, Yund & H epler, 1982). D irection sensitive cells are also found in extra-striate visual areas such as V2 (Zeki, 1978), V3 (Fellem an & van Essen, 1987), and V 4 (D esim one & Schein, 1987; Ferrera et al., 1993). H ow ever, by far the m ost im portant areas so far identified for the processing o f visual m otion are the m iddle tem poral area (M T, also known as V5) and the m edial M ST. In both o f these areas a high proportion o f cells exhibit directional selectivity (A lbright et. al., 1984; M aunsell & V an Essen, 1983; D esim one & Ungerleider, 1986; T anaka et. al., 1986). N ew som e, Britten & M ovshon (1989) dem onstrated a correlation betw een the action o f M T cells and the ability o f a m onkey to discrim inate betw een opposing directions o f m ovem ent. Stim ulating M T cells has been shown to m odulate the decision o f a m onkey when asked to indicate the direction o f m ovem ent that it perceived in a m otion display (Salzm an et al., 1990). The M -pathw ay has been identified as providing the m ajor input to M T by M aunsell et. al., (1990). M aunsell and colleagues selectively blocked the activity o f both M and P processing stream s, finding that the response o f M T neurons was substantially reduced when the M p athw ay w as tem porarily lesioned, b ut rem ain ed largely u naffected by P pathw ay blockade. W hile M T cells are selective for direction o f m otion, and are broadly tuned for speed (M aunsell & van Essen, 1983; Lagae et al., 1993), they are insensitive to either the shape or colour o f the stim ulus (Zeki, 1974). As in

e a rlie r visual areas the rec e p tiv e field size o f M T neurons varies w ith eccentricity, becom ing larger with increasing distance from the fovea (A lbright & D esim one, 1987). M T projects to M ST w here receptive fields are larger in size, and are thought to be especially suited to the detection o f w ide-field m ovem ents (Tanaka, 1998). Such patterns o f flow are those typically caused by the m otion o f the observer through the environm ent, and may take the form o f a rigid translation, expansion / contraction or rotation. Cells selectively responsive to these prototypical patterns are found in dorsal M ST (Sakata et al, 1985; Sakata et. al., 1986; Sakata et al., 1994; Saito et al., 1986; D uffy & W urtz, 1991; O rban et. al, 1992; G raziano et. al., 1994), presum ably allow ing for the coding o f ego m otion, especially when one unusual property o f these cells is considered. ‘Position invariance’ refers to the lack o f sensitivity o f radial and rotary cells to th e lo c u s o f th e ir p r e f e rr e d p a tte rn o f m o tio n . T h e c e n tre o f expansion/contraction or rotation fields does not need to be placed at the centre o f the receptive field in order to elicit strong cell responses. This may give these cells the ability to code the m otion o f the an im al’s body independently o f the gaze direction. Cells in ventral M ST exhibit a different pattern o f responses to those in the dorsal region, one that seem s to be sensitive to the relationship betw een figure and ground. A large proportion o f them register the presence o f sm all stationary stim uli, w ith the sign o f their directional selectivity being contingent on the presence or absence o f a background w ide field m otion (Sugita & Tanaka, 1991; T anaka et al., 1993). T his pattern o f responses gives these neurons the property o f detecting the relativ e m otion betw een o b ject and background (Tanaka, 1998).