cognitive 3 continued

physiology to behaviour

physiology

separation of when and where signals in both the brainstem saccade generator and the superior colliculus

rostral pole: fixation neurones

WHEN. cells within the rostral pole firre during fixations and pause during saccades, cells in the fixation centre link to the omnipause cells in the brainstem saccade generator

pause cells= when.

deep levels of the SC

WHERE.burst neurones, cells activitty prior to saccades, both specific to saccade metrics- buildup and burst cells. burst cells= where

buildup cells

activity increases over time and peaks just before saccade

burst cells

similar to those in the brainstem saccade generator, brief activity just before the saccade

superior colliculus: when and where

balance of activity between the when fixation centre and the where buildup cells. during a fixation, activity in the fixation centre reduces and activity in the buildup cells increases, at a certain threshold, the fixation centre activity stops, burst cells fire and the brainstem saccade generator system is activated and eye movement happens

Findlay and Walker- physiologically plausible framework for saccade generation

level 1= similar to brainstem saccade generator and level 2= similar to superior colliculus. this account is physiologically plausible

the when/ where distinction: physiology

physiological evidence for spearate coding of when and where. Sc fixation centre and BSG pause neurones only code when. Sc deep players and BSG (brainstem saccade generator) burst neurones only code where.

physiological evidence predicts

behavioural studies should show that different factors should influence when and where the eyes move as these are controlled by different mechanisms

behavioural evidence of the when and where distinction

the gap effect: p’s tasl to look at fixation cross, when target appears on screeen look at it as quickly and accurately as you can. 2 conditions: fixation cross disappears before the target appears (gap) and fixation cross remains when target appears (overlap) DV: time taken to initiate saccade from target onset

findings

Saslow- shorter saccade latencies when there is a delay (gap) between fixation point offset and the new target onset i.e. the gap effect can speed when before where is specified. takes longer to move eyes if fixation cross is on screen. faster to respond when gap condition happens

explanations for the gap effect

Ross and Ross- indiicates separate programming of when and where becauswe they can move eyes faster even when they dont know where their eyes are going to move

the when/ where distinction

physiological evidence: separate coding of when and where in the brainstem saccade generator and the superior colliculus. theory: separation of when and where control in Findlay and Walker’s framework

the fixation centre

the gap effect is also in line with evidence for a fixation centre at the rostral pole of the superior colliculus

evidence from the gap effect

Munoz and Wurtz: removal of the fixation spot reduces fixation activity at the Sc rostral pole whilst buildup cell activity is increased, incresing the likelihood of a short latency (express) saccade- demonstarted fixation cells fire during fixations and pause during saccades

modelling the gap effect with physiological plausibility

Findlay and Walker- stimulus offset at fixation reduces activity in the fixate centre, increasing the likelihood of a saccade

distributed population coding

distributed population coding determines the location of the saccade target i.e. where the eyes move. also referred to as salaience maps. large overlapping recpetive fields i.e. each cell represents an average of activity across a large region. locaitons compete with eachother for greatest level of activity . one location always has highest activity/ salience. within SC theres lots of overlapping cells so you can pinpoint which level has highest activity and thats where the eyes will move to

stimulated competition within a salience map- Koch

initially, theres competitiion such that over time one area becomes stronger and stronger so that the other areas lose activation and then tit reaches a threshol;d and one part gets fixated on

salience maps: spatially, not feature, specific

feature maps- selectivity for particular features in aprticular locations

visual salience map: sensory, bottom up- higher salience for more distinctive characteristics

largely driven by low level factors but also input from higher areas

saccadic averaging

activation of 2 close locations can result in averaging and hence a saccade to an intermediate location

saccade averaging: physiological evidence

Robinson- stimulation of 2 locations produces a saccade to the average of the 2 i.e. 2 stimuli can generate a single saccade target, as in global effect. whatever location they stimulate, the eyes will move in that direction. if we stimulate 2 different positions, the eyes will go straight ahead and average in the middle fi the 2 stimuli are close together 

saccadic averaging: behavioural evidence

Findlay: when a saccade is targeted to 2 stimuli at neighboruing positiions the eyes tend to be directed to the centre of gravity between them. modulated by size and brightness of the 2 stimuli

glocal effect- eyes lay in middle fo the 2 targets