code: def _num_units_signal(num_units_vals, bias_val, decay):
      """Signal on the number of units."""
      num_units_vals = np.array(_get_vals_wo_None(num_units_vals))
      return np.exp(-decay * abs(num_units_vals.mean() - bias_val))
  
file_path: dragonfly/dragonfly/nn/syn_nn_functions.py
imports: import numpy
label_desc: Use image node_id 2 for calling a global function with example usage: _num_units_signal(num_units_vals, bias_val, decay) and returns: np