Moving colors in the lime light.

Until recently, investigations of color influences on motion responses in area MT have focused exclusively on red/green input (i.e., derived from signals in the L and M cones). But now, in a triplet of papers appearing in last month's issue of Neuron, a group of vision scientists have reported, for the first time, substantial S cone input to cortical motion processing. Employing color gratings modulated along the blue/yellow dimension (designed to isolate contribution from S cones), these investigators used functional magnetic resonance imaging (fMRI) to demonstrate S cone input to the motion-responsive region of human visual cortex (referred to as MT+) (Wandell et al. 1999xWandell, B.A, Poirson, A.B, Newsome, W.T, Baseler, H.A, Boynton, G.M, Huk, A, Gandhi, S, and Sharpe, L.T. Neuron. 1999; 24: 901–909Abstract | Full Text | Full Text PDF | PubMedSee all ReferencesWandell et al. 1999). These experiments were complemented by neuronal recordings in macaque area MT, demonstrating a clear S cone contribution to directionally selective responses (Seideman et al. 1999xSeideman, E, Poirson, A.B, Wandell, B.A, and Newsome, W.T. Neuron. 1999; 24: 911–917Abstract | Full Text | Full Text PDF | PubMedSee all ReferencesSeideman et al. 1999). In both domains (human fMRI and macaque neurophysiology), however, S cone input to motion was found to be significantly less powerful than light/dark input. Specifically, the contrast sensitivity of MT neurons was approximately ten times lower for blue/yellow, as compared to light/dark, moving gratings. These neural results mirror their psychophysical findings (Dougherty et al. 1999xDougherty, R.F, Press, W.A, and Wandell, B.A. Neuron. 1999; 24: 893–899Abstract | Full Text | Full Text PDF | PubMedSee all ReferencesDougherty et al. 1999; see also Lee and Stromeyer 1989xLee, J and Stromeyer, C.F. J. Physiol. (Lond). 1989; 413: 563–593See all ReferencesLee and Stromeyer 1989) demonstrating impaired motion perception for stimuli modulated along the blue/yellow dimension and are in qualitative agreement with previous results in MT obtained using red/green gratings (6xDobkins, K.R and Albright, T.D. J. Neurosci. 1994; 14: 4854–4870PubMedSee all References, 10xGegenfurtner, K.R, Kiper, D.C, Beusmans, J.M, Carandini, M, Zaidi, Q, and Movshon, J.A. Vis. Neurosci. 1994; 11: 455–466CrossRef | PubMedSee all References).With the popularity of the K pathway on the rise, an exciting prospect is that the observed S cone influences on MT responses reflect functional input from this third visual pathway. Signals from the K pathway could potentially reach area MT from the V1 blobs via V2 (see Merigan and Maunsell 1993xMerigan, W.H and Maunsell, J.H. Annu. Rev. Neurosci. 1993; 16: 369–402CrossRef | PubMedSee all ReferencesMerigan and Maunsell 1993, for details regarding projections between these areas). Alternatively, the direct (yet sparse) connections known to exist between K layers in the LGN and area MT (Figure 2Figure 2; Hendry and Reid 2000xHendry, S.H.C and Reid, R.C. Annu. Rev. Neurosci. 2000; 23: 127–153CrossRef | PubMed | Scopus (298)See all ReferencesHendry and Reid 2000) could contribute an S cone signal. Like the P pathway hypothesis mentioned earlier, functional K pathway input to MT would suggest that the S cone signals carried by this pathway could be used for multiple purposes—to identify the chromatic features of an object (e.g., a blue flower), as well as assist in motion detection. Despite the appeal of this proposal, another possibility should be considered: S cone signals might reach area MT by piggybacking within the M pathway. Although there does exist evidence from retinal anatomy to support this possibility (Calkins 2000xCalkins, D.J. J. Opt. Soc. Am. B. 2000; 17See all ReferencesCalkins 2000), the general consensus from neurophysiological studies is that S cone contribution to M pathway responses is negligible if not altogether absent (e.g., 15xLee, B.B, Martin, P.R, and Valberg, A. J. Physiol. (Lond). 1988; 404: 323–347See all References, 4xDacey, D.M and Lee, B.B. Nature. 1994; 367: 731–735CrossRef | PubMed | Scopus (458)See all References). Certainly, more extensive studies will be required to unequivocally establish the nature of S cone input to MT.In conclusion, there now exists substantial evidence that all three color dimensions—light/dark, red/green, and blue/yellow—influence motion processing revealed perceptually, and in area MT. This color information may reach MT via input from the three subcortical pathways—magnocellular, parvocellular, and koniocellular, respectively. The existence of such input would suggest that color signals within a single pathway may be used for multiple purposes in visual perception—to identify object color, as well as aid in motion detection. Alternatively, or in addition to this possibility, color information along a single dimension may be relayed by multiple visual pathways (e.g., by both parvocellular and magnocellular, in the case of red/green stimuli), but in a different manner by each depending on their more specific functions in visual perception. In any event, there appear to exist ample opportunities for color and motion signals to intermingle in the brain, perhaps even in yet undiscovered ways.*E-mail: kdobkins@ucsd.edu.