Furthermore, when we applied β = 2.75 in Equation 3 to the parameters obtained by fitting the eight normalization conditions (attention directed away from the receptive field), 94% of the variance in average responses was explained for the four attention conditions (attention directed to the receptive field). Therefore, fitting the free parameters of Metformin the model to the normalization conditions alone, then applying β = 2.75 according to Equation 3, was enough to predict the firing rate effects of attention per neuron. Our results show

that a significant portion of the variance in attention modulation across neurons in MT can be attributed to variance in normalization strengths across neurons. Importantly, this correlation is not dependent on the tuning of the neurons to the individual stimuli presented. Even when neurons strongly differentiate between preferred and null stimuli, different neurons respond differently when a null stimulus is added to a preferred stimulus. This variation can be attributed to differences in tuned normalization. For neurons with normalization that is not tuned (α = 1), a null stimulus that does not drive a response will nevertheless be factored into normalization, causing www.selleckchem.com/products/MLN-2238.html them to respond much less when a null stimulus is paired with preferred stimulus. For neurons with highly tuned normalization (α = 0), a null stimulus not only fails to produce

a response but also is effectively prevented

from contributing to normalization, such that the response to the preferred stimulus is unaffected by the addition of a null stimulus in the receptive through field. While many studies have investigated the biophysical mechanisms underlying the normalization mechanism in general (Abbott et al., 1997, Carandini et al., 1997, Carandini et al., 2002, Shadlen and Newsome, 1998, Chance et al., 2002, Mitchell and Silver, 2003, Prescott and De Koninck, 2003, Carandini and Heeger, 1994, Finn et al., 2007, Buia and Tiesinga, 2008, Kouh and Poggio, 2008, Priebe and Ferster, 2008 and Chaisanguanthum and Lisberger, 2011), the biophysical mechanisms underlying tuned normalization are not known. Several reports have shown how normalization can explain the large modulations that are seen when attention is shifted between preferred and null stimuli in the receptive field of a neuron (Boynton, 2009, Lee and Maunsell, 2009 and Reynolds and Heeger, 2009). Because responses to the preferred and null stimuli contribute both to the excitatory drive and also to divisive normalization, relatively modest modulations of the inputs associated with each stimulus are effectively amplified by the normalization mechanism. Strongly tuned normalization effectively removes a null stimulus from normalization and therefore removes the basis for the strong modulations by attention that can occur from shifting attention between preferred and null stimuli.