Prepulse reactivity in prepulse inhibition.

Prepulse inhibition (PPI) is a popular paradigm in sensorimotor gating research. In healthy individuals the weak lead stimulus (i.e., the prepulse) presentation results in a reduction in the startle probe (pulse) elicited response. The motor responses to the prepulses (prepulse reactivity, PPER) were until recently largely ignored in PPI research. There are conflicting reports about prepulse reactivity and startle response modification (SRM) associations; and personality factors relevant to SRM have not been previously examined in prepulse reactivity context. Healthy participants were drawn from university student and staff population. Three paradigms were used: unpredictable stimulus onset, predictable stimulus onset and conscious stimulus processing. The stimuli consisted of 80, 85 & 90dB prepulses and 115dB startle probe separated by 140ms inter-stimulus interval (onset to onset asynchrony). The inter-trial intervals varied between the studies. Startle responses were measured as eye blinks and recorded using surface EMG. All motor responses were quantified according to the same set of rules. Prepulse-elicited motor responses reliably appeared in all the studies and were distinct from spontaneous EMG. Some PPER characteristics exhibited stimulus intensity dependence further proving PPER validity as stimulus-driven response. Prepulse reactivity exhibited significant associations with startle response modification. PPER was a stable tendency; individuals either consistently responded to the weak lead stimuli or did not. Two types of startle response modification appeared under the conditions assumed to elicit maximal inhibition only: classical inhibition (as expected) and paradoxical prepulse facilitation. These appeared in motor responses and in conscious stimulus processing. The propensity towards the paradoxical prepulse facilitation was reduced by efficient prepulse inhibition. PPER and SRM had limited associations with personality factors, sex, or age. The predictable stimulus onset paradigm however highlighted the associations of the defensive startle response and its modification with fear and anxiety. Increased emotionality, regardless of its valence, proved detrimental to sensorimotor gating

The time course of activation of object shape and shape+colour representations during memory retrieval

Little is known about the timing of activating memory for objects and their associated perceptual properties, such as colour, and yet this is important for theories of human cognition. We investigated the time course associated with early cognitive processes related to the activation of object shape and object shape+colour representations respectively, during memory retrieval as assessed by repetition priming in an event-related potential (ERP) study. The main findings were as follows: (1) we identified a unique early modulation of mean ERP amplitude during the N1 that was associated with the activation of object shape independently of colour; (2) we also found a subsequent early P2 modulation of mean amplitude over the same electrode clusters associated with the activation of object shape+colour representations; (3) these findings were apparent across both familiar (i.e., correctly coloured – yellow banana) and novel (i.e., incorrectly coloured - blue strawberry) objects; and (4) neither of the modulations of mean ERP amplitude were evident during the P3. Together the findings delineate the timing of object shape and colour memory systems and support the notion that perceptual representations of object shape mediate the retrieval of temporary shape+colour representations for familiar and novel objects

ERP Shape+colour repetition effects.

<p>(a) Grand average ERPs to same (red line) versus change (black line) stimuli plotted between −100 and 500 msec, with respective topographies for the P2 at peak amplitude; (b) topography associated with the difference in mean amplitude for the same versus change contrast over the highlighted 40 msec epoch for the P2; (c) mean amplitudes across posterior electrode clusters during the P2 as a function of laterality and transformation.</p

ERP Colour effects.

<p>Grand average ERPs to correctly (black line) and incorrectly (red line) coloured objects plotted between −100 and 800 msec with (a) topography associated with the peak amplitude for correctly versus incorrectly coloured objects for the P2; and (b) topography associated with the peak amplitude for correctly versus incorrectly coloured objects for the P3.</p

ERP Shape repetition effects.

<p>(a) Grand average ERPs to new (blue line) versus change (black line) stimuli plotted between −100 and 500 msec, with respective topographies for the N1 at peak amplitude; (b) topography associated with the difference in mean amplitude for the new versus change contrast over the highlighted 40 msec epoch for the N1; (c) mean amplitudes across posterior electrode clusters during the N1 as a function of laterality and transformation.</p

Mean response times (msec) and percentage correct (%) for the coloured-object decision test; values in brackets represent standard error.

<p>Mean response times (msec) and percentage correct (%) for the coloured-object decision test; values in brackets represent standard error.</p