Three-Toed Sloth

Friday Cat Blogging (Son et Lumière Issue of Science Geek Edition)

A lot of work on neural computation just looks at understanding how animals process information coming from a single sense, but of course our ears don't shut off when we look at something, and it's well-established on a behavioral level that you can get more of a response with a noise and a flash than with either alone. Here is a careful paper looking at the neural basis of such multisensory integration in cats.

Terrence R. Stanford, Stephan Quessy, and Barry E. Stein, "Evaluating the Operations Underlying Multisensory Integration in the Cat Superior Colliculus", The Journal of Neuroscience 25 (2005): 6499--6508

Abstract: It is well established that superior colliculus (SC) multisensory neurons integrate cues from different senses; however, the mechanisms responsible for producing multisensory responses are poorly understood. Previous studies have shown that spatially congruent cues from different modalities (e.g., auditory and visual) yield enhanced responses and that the greatest relative enhancements occur for combinations of the least effective modality-specific stimuli. Although these phenomena are well documented, little is known about the mechanisms that underlie them, because no study has systematically examined the operation that multisensory neurons perform on their modality-specific inputs. The goal of this study was to evaluate the computations that multisensory neurons perform in combining the influences of stimuli from two modalities. The extracellular activities of single neurons in the SC of the cat were recorded in response to visual, auditory, and bimodal visual-auditory stimulation. Each neuron was tested across a range of stimulus intensities and multisensory responses evaluated against the null hypothesis of simple summation of unisensory influences. We found that the multisensory response could be superadditive, additive, or subadditive but that the computation was strongly dictated by the efficacies of the modality-specific stimulus components. Superadditivity was most common within a restricted range of near-threshold stimulus efficacies, whereas for the majority of stimuli, response magnitudes were consistent with the linear summation of modality-specific influences. In addition to providing a constraint for developing models of multisensory integration, the relationship between response mode and stimulus efficacy emphasizes the importance of considering stimulus parameters when inducing or interpreting multisensory phenomena.

They start with individual spike-trains, and use those to build post-stimulus time histograms for the spiking rate, but mostly they on the number of spikes evoked in the first 50 ms after the stimulus, which assumes a kind of rate coding. (No attempt at working out a temporal code is made here.) They do, however, do a pretty thorough job of analyzing the rates, including a nice null model of additivity (using the rastergrams), and corrections for spontaneous activity. The abstract does a pretty good job of summarizing the results of this analysis. They use them to suggest a simple and attract model, but dash those hopes a few paragraphs later.

That most multisensory interactions were consistent with simple summation of modality-specific influences suggests that a very basic linear model of the SC might account for many of the current observations. For example, the fact that superadditive interactions were common only in response to combinations of very weak modality-specific stimuli suggests that such superadditivity reflects temporal summation of the [post-synaptic potentials] consequent to near-threshold activity on the auditory and visual input channels. Subadditive interactions were relatively uncommon; however, their correspondence with combinations of the most effective stimuli could likewise be the simple consequence of approaching an intrinsic firing frequency limit of the SC neuron. In principle, a qualitatively similar relationship between computational mode and stimulus efficacy could be produced by an integrate-and-fire model that includes threshold and saturating nonlinearities. [p. 6506]

Alas, as they say, such a nice near-linear model has two fatal flaws. First, it doesn't account for the variability among neurons, and the fact that "few displayed the transition in its entirety from superadditivity to subadditivity". Second, the SC gets input from the cortex as well as from the sense organs, and while blocking cortical input leaves modality-specific responses alone, it eliminates multi-modal enhancement, which is not compatible with the near-linear model.

As you may have guessed, there is no feline cuteness to the paper at all, though I am happy to report that "[a]t the end of an experiment, injection of anesthetics and paralytics was terminated, and animal [sic] was allowed to recover normal respiration and locomotion before being returned to its home cage" (p. 6500).

Friday Cat Blogging; Minds, Brains, and Neurons