It has been additionally suggested that, within the memory pattern, time overestimation denoted by rightward shifts of the psychometric function, might depend on a selective reference memory breakdown following frontal lesions, whereas time underestimation and variability increase, that are mostly subsequent to medio-temporal lesions, might be interpreted as working memory deficits.
The size discrimination threshold was given by the standard deviation of the psychometric function (i.e. the slope of the function).
All training was performed at the 85% correct level of the psychometric function estimated by the ZEST algorithm.
By measuring changes in slope of the psychometric function we can obtain estimates of how discriminable leftwards versus rightwards displacement are.
Gap-detection sensitivity was measured by the gap length at which the psychometric function for each session reached the threshold, defined here as d′ = 1.
The size of the horizontal shift of the psychometric function induced by electrical microstimulation was re-calculated as described in the previous section, separately for large and small reward trials.
(C ) Model simulations illustrate that the slope of the psychometric function, fitted by a simple logistic model of bounded accumulation (Equation 3a ), can be affected by changes in either parameter k or B ; for example, an increase in either parameter will steepen the slope, indicating improved performance.
However, it should be noted that the psychometric function is determined by a stochastic process based on many binary responses.
A threshold for 75% correct responses was determined by maximum likelihood estimation of the parameters of the psychometric function.
To normalise the effect of microstimulation across sites with different associated threshold due to stimulus eccentricity (see Figure 8), the horizontal shift of the psychometric function in each reward condition was divided by the psychometric threshold for that reward condition that is, the standard deviation (s.d). of the fitted cumulative Gaussian (Equation 1a; Uka and DeAngelis 2006).
