This is THE fundamental system for all behavior “seeking/craving/getting ” behavior and all business, duh.
“…dopamine may act as a filter for selecting particular inputs, and thereby exert selective effects on corticostriatal inputs that underlie various behaviors.”
- The novelty response of DA neurones habituates rapidly when a sensory stimulus is repeated in the absence of behaviourally rewarding consequences.
- A phasic DA response will emerge following the presentation of a neutral sensory stimulus that predicts a primary reward. Under these conditions the DA responses to the predicted reward gradually diminish.
- When a predicted reward is omitted, a reliable depression in the spontaneous activity of the DA neurones occurs 70-100 ms after the time of expected reward delivery.
- Although dopamine neurones have reliable responses to reward-related stimuli they also exhibit strong phasic responses to unexpected sensory events that have no obvious appetitive reinforcement consequences
- Despite reward-related stimuli coming in all sorts of shapes and sizes, the phasic dopamine signal is highly stereotyped (latency 100 ms, duration) and largely independent of animal species, stimulus modality, and perceptual complexity of eliciting events
- The 100 ms response latency of dopaminergic neurones is reliably shorter than the latency of the gaze-shift that brings the unexpected event onto the fovea for detailed analysis by cortical visual systems. Necessarily this means that dopamine responses are triggered as a consequence of limited pre-attentive, pre-saccadic sensory processing.
- Recent evidence indicates that the sensory inputs to dopaminergic neurones derive largely, if not exclusively as a consequence of early, subcortical sensory processing (Redgrave and Gurney 2006). In the case of vision, the midbrain superior colliculus is configured to indicate where an unexpected event is rather than what it is (Wurtz and Albano 1980). Perhaps it is no coincidence that, in almost all studies showing phasic dopamine signals can signal reward prediction errors (Schultz 2006), the economic values predicted by the conditioned stimuli are correlated with the spatial location of stimulus presentation. It therefore remains to be determined whether dopamine neurones can signal continuous values of reward prediction errors in real world conditions where unexpected events are both temporally and spatially unpredictable.
….the hypothesised errors in reward prediction signalled by phasic dopamine activity are presumed teaching signals for appetitive learning and ensure that actions maximising the future acquisition of reward are selected more often.
However, recent evidence from studies that have identified sources of short-latency sensory input to midbrain dopaminergic neurones suggests that, in real world conditions where unexpected stimuli are both temporally and spatially unpredictable, the identity of unexpected events (and hence their reward value) will be determined after, rather than before the time of phasic dopaminergic signalling
Figure 8: Potentially converging inputs to the dorsal striatum at the time of an unpredicted biologically salient visual event.
A. Phasic sensory: Two separate short-latency representations of the visual event could converge on striatal circuitry: (i) retino-tecto-thalamo-striatal projections will provide a phasic sensory-related glutamatergic input (red arrows) ; and (ii) retino-tecto-nigro-striatal projections will provide a phasic dopaminergic input (orange arrows) .
B. Contextual: Striatal neurones are sensitive to experimental context. Multidimensional contextual afferents are likely to originate from cerebral cortex, limbic structures (hippocampus and amygdala) and the thalamus (Blue arrows).
C. Motor copy: Branched pathways from motor cortex and subcortical sensorimotor structures (e.g. superior colliculus) reach the striatum directly (cortex) or indirectly via the thalamus (subcortical structures). Motor-related projections could provide the striatum with a running, multidimensional record (motor efference copy) of commands relating to ongoing goals/actions/movements (green arrows).
In the light of these considerations, it has been suggested that”
- short-latency signalling by dopaminergic neurones may be suited more to reinforcing a form of learning with less stringent perceptual requirements
- Specifically, short-latency dopamine reinforcement signals could promote the discovery of agency (i.e. those initially unpredicted events that are caused by the agent) and subsequent identification of critical causative actions, irrespective of the outcome’s economic value.
- This hypothesis is based on the sensory and motor signals likely to be present in target structures (principally the striatum) at the time of the precisely timed phasic dopamine response.
- The role of dopamine in this scheme is to promote the reselection of components of behaviour and context that immediately precede unpredicted sensory events. When the animal/agent is the cause of an event, repeated trials should enable the basal ganglia to converge on behavioural and contextual components that are critical for eliciting it, leading to the emergence of a novel action.
Figure 9: The relative timing of proposed inputs to the dorsal striatum could be used to determine the source of agency.
A. Event caused by subject: Whenever the subject is the cause of an unpredicted sensory event, relevant components of the multidimensional contextual (blue) and motor efference copy (green) inputs will directly precede the near simultaneous short-latency glutamateric sensory input from the thalamus (red) and the phasic dopaminergic input from substantia nigra (orange).
B. Event caused by external source. When no relevant motor copy inputs precede the phasic sensory inputs (glutamateric and dopaminergic), the unpredicted event is likely to have been caused by an external source. This circuitry could enable the system to determine those events in the external world for which it is responsible. The colour coding of the functional inputs to the striatum is maintained from Figure 8.