Similar vs Dissimilar: Two Generalization Systems, Two Substrates

Fundamentally, there are only two processes by which an organism’s brain will determine that these stimuli are grouped together: one way being that they appear similar to each other, and the other being that they are both associated with something else. These two processes are “perceptional generalization” and “learning-based generalization,” respectively.

So, Perceptual generalization emerges directly from the structure of the sensory cortex. The patterns representing a 550 Hz tone and a 560 Hz tone activate largely overlapping groups of neurons in the primary auditory cortex, and these representations use the same set of input neurons. Now, having learned the relationship that exists between a particular tone (550 Hz) and a particular outcome, we observe that the pattern of neurons representing the second tone gets to enjoy the association for free, as many of its neurons have already learned a partial association. This becomes the basis of the bell-shaped gradient of perceptual generalization, with peak activity at the conditioned stimulus as well as a decline proportional to physical distance.

The physiological correlate of this phenomenon is more specific still. In experiments carried out by Bakin and Weinberger, pairing a 2500 Hz tone with foot shock causes retuning of the A1 receptive fields to move closer to the frequency of the conditioned stimulus. This reflects how the brain pays attention to what’s important by tuning itself appropriately. The cue for paying attention is provided by the nucleus basalis of Meynert, whose acetylcholine release kind of permeates cortical plasticity. Kilgard and Merzenich showed that a tone paired with stimulation of the nucleus basalis remapped A1 just like foot shock. Acetylcholine doesn’t carry information, it simply says, ‘Pay attention’.

Now, notably learning-based generalization works differently. Two stimuli without any common physical properties, such as a tone and a light, may be considered equivalent after the organism has learned that they lead to the same consequence or were present together in some sequence. Sensory preconditioning serves as a paradigm case, in which tone-light association and subsequent light-airpuff association result in a blink response to tone only. Moreover, acquired equivalence and latent inhibition are in the same category of phenomena, but they all vanish once there is damage to the entorhinal cortex (Coutureau et al., 2002), while perceptual generalization is immune. It is the hippocampus that integrates various sensations and forms one coherent experience. It is the hippocampus that performs the function of an AND-gate for configural processes, which cannot be realized using a one-layered associator.

References

Bakin, J. S., & Weinberger, N. M. (1990). Classical conditioning induces CS-specific receptive field plasticity in the auditory cortex of the guinea pig. Brain Research, 536(1–2), 271–286. https://doi.org/10.1016/0006-8993(90)90035-A

Coutureau, E., Lena, I., Daugé, V., & Di Scala, G. (2002). The entorhinal cortex–nucleus accumbens pathway and latent inhibition: A behavioral and neurochemical study in rats. Behavioral Neuroscience, 116(1), 95–104. https://doi.org/10.1037/0735-7044.116.1.95

Kilgard, M. P., & Merzenich, M. M. (1998). Cortical map reorganization enabled by nucleus basalis activity. Science, 279(5357), 1714–1718. https://doi.org/10.1126/science.279.5357.1714

-Sanj