Basic information processing and higher cognition: does the mammalian cerebral cortex deal with them at different hierarchical levels?

The plasticity of pyramidal neurons in the behaving brain

Neurons are plastic. That is, they change their activity according to different behavioural conditions. This endows pyramidal neurons with an incredible computational power for the integration and processing of synaptic inputs. Plasticity can be investigated at different levels of investigation within a single neuron, from spines to dendrites, to synaptic input. Although most of our knowledge stems from the in vitro brain slice preparation, plasticity plays a vital role during behaviour by providing a flexible substrate for the execution of appropriate actions in our ever-changing environment. Owing to advances in recording techniques, the plasticity of neurons and the neural networks in which they are embedded is now beginning to be realized in the in vivo intact brain. This review focuses on the structural and functional synaptic plasticity of pyramidal neurons, with a specific focus on the latest developments from in vivo studies. This article is part of a discussion meeting issue 'Long-term potentiation: 50 years on'.

Competitive and noncompetitive NMDA antagonist effects in rats trained to discriminate lever-press counts

The glutamate activated N-methyl-D-aspartate (NMDA) receptor may play a role in short-term memory processing. Among the evidence for this is that NMDA antagonists can impair accuracy in fixed consecutive number (FCN) tasks. This study was designed to further characterize this effect by examining NMDA antagonists differing in their cellular mechanisms of action. Rats were trained to respond under an FCN operant schedule, which required eight presses on one lever (counting lever) before one press at an alternate lever (reinforcement lever) would produce food reinforcement. The effects of three noncompetitive [MK-801 (0.01-0.56 mg/kg); phencyclidine (0.3-3.0 mg/kg); memantine (1-10 mg/kg)] and two competitive [SDZ EAA 494 (0.3-3.0 mg/kg) and NPC 17742 (2.0-16 mg/kg)] NMDA antagonists were analyzed. MK-801 and phencyclidine decreased accuracy at doses not reducing response rates. Memantine, and both of the competitive antagonists, also reduced accuracy, but did so only at doses that markedly reduced response rates. These results suggest that both the affinity and the site bound on the NMDA glutamate receptor by antagonists can determine their effects on FCN performance. Subsequent studies investigated whether SCH 23390, a dopamine D1 receptor antagonist, and NMDA could modulate the effects by phencyclidine and SDZ EAA 494, respectively, on FCN performance.

Perceptual learning in primary and secondary motion vision

Specific improvements of perceptual capabilities with practise are thought to give some clues about cortical plasticity and the localisation of cortical processing. In the present study, perceptual learning is used as a paradigm to separate mechanisms underlying the perception of different classes of motion stimuli. Primary motion stimuli (phi-motion), are characterised by displacements of the luminance distribution. However, for secondary motion stimuli the movement is not accompanied by a corresponding luminance shift. Instead, moving objects are defined by their temporal frequency composition (mu-motion) or by motion itself (theta-motion). On theoretical grounds, the perception of secondary motion requires a higher degree of nonlinearity in the processing stream than the perception of primary motion but debate continues as to whether there might be a unique mechanism underlying the perception of both motion classes. In a large group of subjects, coherence thresholds for direction discrimination in random dot kinematograms of phi-, mu-, and theta-motion were repeatedly measured in a staircase paradigm. Training effects were found on different timescales, within short sessions containing multiple staircases and over training periods of several months. They were fairly stable over long breaks without testing. When subjects were trained with two different motion stimuli in a sequence, an asymmetry in the transfer of perceptual learning was revealed: sensitivity increases achieved during practise of theta-motion are largely transferred to phi-motion, but theta-motion perception does not profit from prior exposure to phi-motion. This finding supports the view derived from modelling of motion processing that there must be at least partially separate systems. A primary motion detection mechanism falls short of discriminating direction in secondary motion stimuli, whereas a mechanism able to extract secondary motion will be inherently sensitive to primary motion.

Gating of neuronal responses in macaque primary visual cortex by an attentional spotlight

Neuronal responses were recorded from the striate cortex of monkeys trained to perform visual discrimination at locations in the visual field to which their attention was drawn. A subset of neurons showed vigorous responses to visual stimuli for trials in which the monkey was directing its attention to the respective receptive field location. In trials where attention is directed elsewhere, responses to the same stimuli were significantly reduced. In some cells the early response component was not modulated by attention, but later components were affected by the locus of attention. The results suggest the operation of a feedback in the paradigm that spotlights a topographically restricted area of V1 for further processing at higher levels.