P.Christiaan Klink,
PhD candidate
Functional Neurobiology & Helmholtz Institute, Utrecht University, 3584 CH Utrecht, the Netherlands
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Re: Attention vs. Contrast for the Single Neuron: Does the Analogy Hold?
P.C.Klink{at}uu.nl P.Christiaan Klink
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Do the findings of Lee et al. mean that the common analogy of attention as an effective contrast enhancer breaks down at the level of the single neuron? Unfortunately a few interesting issues remain undiscussed.
A possible explanation for the demonstrated difference in effect on response latency between contrast and attention could be that an initial contrast-induced speed-up of the neuronal response in the retina (Gawne et al, 1996) suffices to explain latency changes in V4. The common analogy of attention as an effective contrast enhancer may still hold for all cortical areas, but at the retina there is only real contrast and apparently real contrast shortens response latencies of retinal cells. Even though effects of attention have been reported as early as the lateral geniculate nucleus (O'Conner et al., 2002) or thalamic reticular nucleus (McAlonan et al., 2006), they are unlikely to play a role at the level of the retina. If contrast only modulates latency when visual information enters our perceptual system, endogenous attention will never be able to influence them since it is by definition arising from -and limited to- 'within' the system. A comparison between contrast-induced latency modulations in different stages of visual processing might reveal whether these modulations only occur in the retina or if additional modulations also occur higher up the visual cortical hierarchy.
Other studies have suggested that response latencies might act as a temporal label for the binding of stimulus features (Gawne et al, 1996). This would explain why the latencies would be set as they enter the visual processing system. Cortical areas might just inherit the labels as the signal moves along the visual cortical hierarchy. A comparable rationale would also apply to neuronal response latencies as discriminatory labels to distinguish stimulus saliency from an endogenous re-evaluation of sensory information.
Another issue is that the reported effects of contrast on response magnitude are much larger than the effect of attention. If response magnitude and latency are related in the same way for attention and contrast it could be the case that the extent to which attention influences response latencies is simply too small to reach statistical significance. The authors performed a regression analysis revealing that changes in response latency caused by attention were much smaller than those caused by contrast with a comparable change in response magnitude (Lee et al. fig. 4). However, another recent study reports attention-driven latency changes in V4 neurons that may be smaller than contrast-induced latency shifts, but are nevertheless significant (Sundberg et al., 2007).
In summary, this study places an important footnote to the use of analogies in scientific research in general. Attention in particular may be very similar to contrast gain in many aspects, but at the detailed level of the single neuron this analogy partly breaks down. Instead of just rendering the analogy useless, these findings shed an interesting new light on the possible differences in neuronal processing of real and effective contrast in visual perception.
References
Gawne, T. J.; Kjaer, T. W. & Richmond, B. J. (1996), 'Latency: another potential code for feature binding in striate cortex.', J Neurophysiol 76(2), 1356--1360.
Lee, J.; Williford, T. & Maunsell, J. H. R. (2007), 'Spatial attention and the latency of neuronal responses in macaque area V4.', J Neurosci 27(36), 9632--9637.
McAlonan, K.; Cavanaugh, J. & Wurtz, R. H. (2006), 'Attentional modulation of thalamic reticular neurons.', J Neurosci 26(16), 4444--4450.
O'Connor, D. H.; Fukui, M. M.; Pinsk, M. A. & Kastner, S. (2002), 'Attention modulates responses in the human lateral geniculate nucleus.', Nat Neurosci 5(11), 1203--1209.
Sundberg, K.A.; Mitchell, J.F.; Reynolds, J.H. (2007), 'Attention-dependent reduction in response latency in area V4', Program No. 177.10. 2007 Neuroscience Meeting Planner. San Diego, CA: Society for Neuroscience, 2007. Online. |
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Attention vs. Contrast for the Single Neuron: Does the Analogy Hold?
