Computational Structure of the Cerebellar Molecular Layer

Abstract

Theories regarding the computational structure of the cerebellar molecular layer have been more strongly influenced by biological data than is the case for many and perhaps even most other vertebrate brain regions. Yet, or perhaps for this reason, the computational significance of this circuitry remains a very active subject of discussion and debate, with even basic assumptions regarding the physiological organization of this circuit now being questioned. Specifically, while most current models of molecular layer circuitry assume that parallel fiber excitatory synaptic input directly drives Purkinje cells to fire, there is growing experimental evidence that this might not be the case. Recent model-based efforts to better understand the functional role of the parallel fiber system suggest that molecular layer inhibition may play a larger and more complex physiological role than previously realized. In turn, this reanalysis of the role of inhibition in cerebellar cortical circuitry suggests that the cerebellar molecular layer may implement a sensory-related contextual algorithm, rather than either a timing or associative learning function previously assumed to provide the basis for a cerebellar involvement in motor coordination.

Consensus Paper: The Role of the Cerebellum in Perceptual Processes

“…At present, most explanations for cerebellar involvement in non-motor-related behaviors assume that evolution has adopted cerebellar motor control computational mechanisms to non-motor tasks [ 58 , 73 , 92 , 211 , 220 , 223 – 226 ], including, for example, a presumed general role for the cerebellum in timing not only of muscle activations during movement but also of sensory perception [ 98 , 227 ]. While our analysis of cerebellar cortical circuitry questions the circuitry-based evidence for the original timing hypothesis [ 205 ], we do expect that any disruption in sensory data acquisition control may very well be particularly apparent with tasks involving precise timing (see the section by Dr. Ivry, “ Sensory Processing and the Cerebellum: Timing (R.B. Ivry) ”).…”

“…While a model-based re-analysis of cerebellar cortical networks also supports this hypothesis [ 205 ], this review will focus on supporting experimental results from more behavioral including human studies. First in a series of imaging experiments, we demonstrated, as the hypothesis predicts, that activity in the human cerebellum [ 51 , 206 ] and related structures [ 207 ] is substantially greater when fingers are used in a tactile discrimination task.…”

“…Finally, while human psychophysical and imaging studies, properly designed, can test functional hypotheses, linking these hypotheses to actual physical computational mechanisms still requires the use of animal models [ 205 ]. While the large majority of animal studies exploring the functional significance of the extensive sensory projections to the cerebellum continue to frame the results in the context of traditional motor control theories [ 216 ], a recent behavioral study in rats has demonstrated that optogenetic stimulation of the cerebellum specifically disrupts the use of the whiskers during active touch [ 217 ].…”

Effects of Attention and Perceptual Uncertainty on Cerebellar Activity During Visual Motion Perception

“…Our results suggest that the cerebellum contributes to the process of visual motion detection and discrimination, but that it does not play a central role in the voluntary sustained allocation of attention to motion. We propose that cerebellar activity under high perceptual uncertainty reflects the operation of regulatory processes that coordinate the acquisition of sensory data [6][7][8] …”

“…A possible explanation for the cerebellum's role in visual motion perception is that it monitors and adjusts sensory data acquisition processes in cortical visual areas, resulting in increased sensitivity to visual motion signals [6][7][8]. Under conditions of severely degraded or ambiguous visual input, these cerebellar regulatory processes could be engaged to facilitate the detection of moving targets.…”