Computational model of biological object recognition The human brain can achieve a remarkable ability to recognize visual patterns in an Invariant, selective and fast manner. The human visual system is quite powerful. It has an exquisite selectivity that allows us to distinguish among very similar objects. In addition, iy also allows Robustness (Invariance) to transformations.

In the model (Ventral stream pathway and architecture proposed by Poggio's group) shown in the image below, why the model is selective in S layers and invariant in C layers ?
I didn't understand well why the S layer achieves selectivity and C layer achieves invariance. Please can someone explain to me in details why the model is selective in S and Invariance in C layers?

And who know the HMAX model, the model is called "Hierarchical Model and X". why X? what was meant by "X" ?


This figure is from Serre et al.'s A quantitative theory of immediate visual recognition. Prog Brain Res. 2007.

Any help will be very very appreciated.
The question is resolved :

The name HMAX ( Hierarchical Model And X ) was coined by “Mike Tarr”, who wrote the “News and Views” accompanying the paper in Nature Neuroscience.

What was meant by Hierarchical?

The model has a hierarchical architecture : it contains different stages (layers). 

-- Increase in receptive field size and complexity in unit feature preferences along the
   hierarchy.

What was meant by X?

I think it is the pooling operation (it refers to the MAX).

Selectivity in S layer

The activity of each S unit is highest if all the afferents have the specified values.

Invariance in C layer

In contrast to S units, the C cells pool over their afferents, and the response is high if 
one of the afferents is high. This increases invariance if the afferents are tuned to the 
same feature, but at different positions or scales.